ISO/IEC 8825-4:2021

Information technology — ASN.1 encoding rules — Part 4: XML Encoding Rules (XER)

This document specifies a set of basic XML Encoding Rules (BASIC-XER) that may be used to derive a transfer syntax for values of types defined in Rec. ITU-T X.680 | ISO/IEC 8824-1 and Rec. ITU-T X.681 | ISO/IEC 8824-2. This document also specifies a set of Canonical XML Encoding Rules (CXER) which provide constraints on the basic XML Encoding Rules and produce a unique encoding for any given ASN.1 value. This document further specifies a set of extended XML Encoding Rules (EXTENDED-XER) which adds further encoders options, and also allows the ASN.1 specifier to vary the encoding that would be produced by BASIC-XER. It is implicit in the specification of these encoding rules that they are also used for decoding. The encoding rules specified in this document : – are used at the time of communication; – are intended for use in circumstances where displaying of values and/or processing them using commonly available XML tools (such as browsers) is the major concern in the choice of encoding rules; – allow the extension of an abstract syntax by addition of extra values for all forms of extensibility described in Rec. ITU-T X.680 | ISO/IEC 8824‑1. This document also specifies the syntax and semantics of XER encoding instructions, and the rules for their assignment and combination. XER encoding instructions can be used to control the EXTENDED-XER encoding for specific ASN.1 types.

Technologies de l'information — Règles de codage ASN.1 — Partie 4: Règles de codage XML (XER)

General Information

• Status: Published
• Publication Date: 29-Jun-2021
• ICS: 35.100.60 - Presentation layer
• Technical Committee: ISO/IEC JTC 1/SC 6 - Telecommunications and information exchange between systems
• Drafting Committee: ISO/IEC JTC 1/SC 6/WG 10 - Directory, ASN.1 and Registration
• Current Stage: 6060 - International Standard published
• Start Date: 30-Jun-2021
• Due Date: 02-Jul-2023
• Completion Date: 30-Jun-2021

Relations

• Revises: ISO/IEC 8825-4:2015 - Information technology — ASN.1 encoding rules: XML Encoding Rules (XER) — Part 4:
• Effective Date: 15-Aug-2020

Standards Content (Sample)

INTERNATIONAL ISO/IEC
STANDARD 8825-4
Fourth edition
2021-06
Information technology — ASN.1
encoding rules —
Part 4:
XML Encoding Rules (XER)
Technologies de l'information — Règles de codage ASN.1 —
Partie 4: Règles de codage XML (XER)
Reference number
©
ISO/IEC 2021
© ISO/IEC 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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ii © ISO/IEC 2021 – All rights reserved

Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that are
members of ISO or IEC participate in the development of International Standards through technical
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ISO and IEC technical committees collaborate in fields of mutual interest. Other international
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Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights. Details
of any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents) or the IEC list of patent
declarations received (see patents.iec.ch).
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For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT)
see www.iso.org/iso/foreword.html. In the IEC, see www.iec.ch/understanding-standards.
This document was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,
Subcommittee SC 6, Telecommunications and information exchange between systems, in collaboration
with ITU-T. The identical text is published as ITU-T X.693 (02/2021).
This fourth edition cancels and replaces the third edition (ISO/IEC 8825-4:2015), which has been
technically revised. It also incorporates ISO/IEC 8825-4:2015/Cor 1:2018.
A list of all parts in the ISO/IEC 8825 series can be found on the ISO and IEC websites.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html and www.iec.ch/national-
committees.
© ISO/IEC 2021 – All rights reserved iii

CONTENTS
Page
1 Scope . 1
2 Normative references . 1
2.1 Identical Recommendations | International Standards . 1
2.2 Additional references . 2
3 Definitions . 2
3.1 ASN.1 Basic Encoding Rules (BER) . 2
3.2 Additional definitions . 2
4 Abbreviations . 4
5 Encodings specified by this Recommendation | International Standard . 4
6 Encoding instructions specified by this Recommendation | International Standard . 5
7 Conformance . 5
8 Basic XML encoding rules . 5
8.1 Production of a complete BASIC-XER encoding . 5
8.2 The XML prolog . 6
8.3 The XML document element . 6
8.4 Encoding of the EXTERNAL type . 7
8.5 Encoding of the open type . 7
8.6 Decoding of types with extension markers . 7
9 Canonical XML encoding rules . 7
9.1 General rules for canonical XER. 7
9.2 Real values . 8
9.3 Bitstring value . 8
9.4 Octetstring value . 8
9.5 Sequence value . 8
9.6 Set value . 8
9.7 Set-of value . 8
9.8 Object identifier value . 9
9.9 Relative object identifier value . 9
9.10 GeneralizedTime . 9
9.11 UTCTime . 9
9.12 Open type value. 10
9.13 The TIME type and the useful time types . 10
10 Extended XML encoding rules . 10
10.1 General . 10
10.2 EXTENDED-XER conformance . 11
10.3 Structure of an EXTENDED-XER encoding . 13
11 Notation, character set and lexical items used in XER encoding instructions . 13
12 Keywords . 14
13 Assigning an XER encoding instruction to an ASN.1 type using a type prefix . 15
14 Assigning an XER encoding instruction using an XER encoding control section . 17
14.1 The encoding instruction assignment list . 17
© ISO/IEC 2021 – All rights reserved
Rec. ITU-T X.693 (02/2021) iii

Page
14.2 Identification of the targets for an XER encoding instruction using a target list . 18
15 Multiple assignment of XER encoding instructions . 23
15.1 Order in which multiple assignments are considered . 23
15.2 Effect of assigning a negating encoding instruction . 24
15.3 Multiple assignment of encoding instructions with multiple categories . 24
15.4 Multiple assignment of XER encoding instructions of the same category . 24
15.5 Permitted combinations of final encoding instructions . 25
16 XER encoding instruction support for XML namespaces and qualified names . 26
17 Specification of EXTENDED-XER encodings . 27
17.1 The XML document element . 28
17.2 The "TypeNameOrModifiedTypeName" production . 28
17.3 The "AttributeList" production . 28
17.4 The "ExtendedXMLValue" production . 28
17.5 The "ExtendedXMLChoiceValue" production . 30
17.6 The "ExtendedXMLSequenceValue" and "ExtendedXMLSetValue" productions . 30
17.7 The "ExtendedXMLSequenceOfValue" and "ExtendedXMLSetOfValue" productions . 31
17.8 The "ModifiedXMLIntegerValue" production . 32
17.9 The "ModifiedXMLRealValue" production . 32
18 The ANY-ATTRIBUTES encoding instruction . 33
18.1 General . 33
18.2 Restrictions . 34
18.3 Effect on encodings . 34
19 The ANY-ELEMENT encoding instruction . 35
19.1 General . 35
19.2 Restrictions . 35
19.3 Effect on encodings . 36
20 The ATTRIBUTE encoding instruction . 37
20.1 General . 37
20.2 Restrictions . 37
20.3 Effect on encodings . 37
21 The BASE64 encoding instruction . 39
21.1 General . 39
21.2 Restrictions . 39
21.3 Effect on encodings . 39
22 The DECIMAL encoding instruction . 40
22.1 General . 40
22.2 Restrictions . 40
22.3 Effect on encodings . 40
23 The DEFAULT-FOR-EMPTY encoding instruction . 41
23.1 General . 41
© ISO/IEC 2021 – All rights reserved
iv Rec. ITU-T X.693 (02/2021)
Page
23.2 Restrictions . 41
23.3 Effect on encodings . 42
24 The ELEMENT encoding instruction . 43
24.1 General . 43
24.2 Restrictions . 43
24.3 Effect on encodings . 43
25 The EMBED-VALUES encoding instruction . 43
25.1 General . 43
25.2 Restrictions . 43
25.3 Effect on encodings . 44
26 The GLOBAL-DEFAULTS encoding instruction . 44
26.1 General . 44
26.2 Restrictions . 45
26.3 Effect on encodings . 45
27 The LIST encoding instruction . 45
27.1 General . 45
27.2 Restrictions . 45
27.3 Effect on encodings . 46
28 The NAME encoding instruction . 46
28.1 General . 46
28.2 Restrictions . 47
28.3 Effect on encodings . 48
29 The NAMESPACE encoding instruction . 48
29.1 General . 48
29.2 Restrictions . 49
29.3 Effect on encodings . 49
30 The PI-OR-COMMENT encoding instruction . 50
30.1 General . 50
30.2 Restrictions . 50
30.3 Effect on the encodings . 50
31 The TEXT encoding instruction . 51
31.1 General . 51
31.2 Restrictions . 51
31.3 Effect on encodings . 52
32 The UNTAGGED encoding instruction . 52
32.1 General . 52
32.2 Restrictions . 53
32.3 Effect on encodings . 53
33 The USE-NIL encoding instruction . 54
33.1 General . 54
33.2 Restrictions . 54
33.3 Effect on encodings . 55
34 The USE-NUMBER encoding instruction . 55
34.1 General . 55
34.2 Restrictions . 55
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Rec. ITU-T X.693 (02/2021) v
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34.3 Effect on encodings . 55
35 The USE-ORDER encoding instruction . 56
35.1 General . 56
35.2 Restrictions . 56
35.3 Effect on encodings . 57
36 The USE-QNAME encoding instruction . 57
36.1 General . 57
36.2 Restrictions . 58
36.3 Effect on encodings . 58
37 The USE-TYPE encoding instruction . 58
37.1 General . 58
37.2 Restrictions . 58
37.3 Effect on encodings . 59
38 The USE-UNION encoding instruction . 59
38.1 General . 59
38.2 Restrictions . 59
38.3 Effect on encodings . 60
39 The WHITESPACE encoding instruction . 61
39.1 General . 61
39.2 Restrictions . 61
39.3 Effect on encodings . 61
40 Identification of the encoding rules . 62
Annex A Examples of BASIC-XER and CXER encodings . 63
A.1 ASN.1 description of the record structure . 63
A.2 ASN.1 description of a record value . 63
A.3 Basic XML representation of this record value . 63
A.4 Canonical XML representation of this record value . 64
Annex B – Partial XML content and deterministic encodings . 65
B.1 Partial XML content . 65
B.2 Recommended restrictions on encodings producing partial XML element content . 65
Annex C – Examples of extended-XER encodings using XER encoding instructions . 68
C.1 Introduction . 68
C.2 Simple examples . 68
C.3 More complex examples . 69

© ISO/IEC 2021 – All rights reserved
vi Rec. ITU-T X.693 (02/2021)
Introduction
Rec. ITU-T X.680 | ISO/IEC 8824-1, Rec. ITU-T X.681 | ISO/IEC 8824-2, Rec. ITU-T X.682 | ISO/IEC 8824-3, Rec.
ITU-T X.683 | ISO/IEC 8824-4 together describe Abstract Syntax Notation One (ASN.1), a notation for the definition
of messages to be exchanged between peer applications.
This Recommendation | International Standard defines encoding rules that may be applied to values of ASN.1 types
defined using the notation specified in Rec. ITU-T X.680 | ISO/IEC 8824-1 and Rec. ITU-T X.681 | ISO/IEC 8824-2.
Application of these encoding rules produces a transfer syntax for such values. It is implicit in the specification of these
encoding rules that they are also to be used for decoding.
There is more than one set of encoding rules that can be applied to values of ASN.1 types. This Recommendation |
International Standard defines three sets of encoding rules that use the Extensible Markup Language (XML). These
encoding rules all produce an XML document compliant to W3C XML 1.0. The first set is called the Basic XML
Encoding Rules (BASIC-XER). The second set is called the Canonical XML Encoding Rules (CANONICAL-XER, or
CXER) because there is only one way of encoding an ASN.1 value using these encoding rules. (Canonical encoding
rules are generally used for applications using security-related features such as digital signatures.) The third set is
called the extended XML Encoding Rules (EXTENDED-XER). The extended XML Encoding Rules allow additional
encoders options, and take account of encoding instructions that specify variations of the BASIC-XER encodings in
order to support specific styles of XML documents (see below). The extended XML Encoding Rules are not canonical,
and there is no canonical form for these rules defined in this Recommendation | International Standard.
There are many aspects of an XML representation of data (such as the use of XML attributes instead of child elements,
or the use of white-space delimited lists) whose use is a matter of style and XML designer choice. If a type defined in
an ASN.1 specification is encoded by BASIC-XER or by CXER, then there is a single fixed style used for the XML
representation, with no user control of stylistic features. This Recommendation | International Standard specifies the
syntax and semantics of XER encoding instructions which specify the stylistic features of the XML in an EXTENDED-
XER encoding. XER encoding instructions can also be used to determine the possible insertion of XML processing
instructions in an EXTENDED-XER encoding. XER encoding instructions are ignored by BASIC-XER and by CXER,
but are used by EXTENDED-XER.
NOTE – "Stylistic features", such as use of attributes or white-space delimited lists, can also affect the size of an encoding and
the ease with which it can be processed, so use of such features is not just a matter of style. Where such issues are important,
EXTENDED-XER with encoding instructions may be preferred over BASIC-XER or CXER.
Clause 8 specifies the BASIC-XER encoding of ASN.1 types.
Clause 9 specifies the CXER encoding of ASN.1 types.
Clause 10 specifies the EXTENDED-XER encoding of ASN.1 types, referencing later clauses which define the XER
encoding instructions.
Clauses 11 to 14 list and categorize the XER encoding instructions and specify the syntax for their assignment to an
ASN.1 type or component using either an XER type prefix (see Rec. ITU-T X.680 | ISO/IEC 8824-1, 31.3) or an XER
encoding control section (see Rec. ITU-T X.680 | ISO/IEC 8824-1, clause 54).
Clause 15 defines the order of precedence if XER encoding instructions are present in both an XER type prefix and in
an XER encoding control section.
Clause 16 specifies the XER encoding instruction support for XML namespaces when using EXTENDED-XER.
Clause 17 specifies EXTENDED-XER encodings.
Clauses 18 to 39 specify:
a) the syntax of each XER encoding instruction used in a type prefix or an XER encoding control section;
b) restrictions on the XER encoding instructions that can be associated with a particular ASN.1 type
(resulting from inheritance and multiple assignments);
c) modifications to the XER encoding rules that are required in an EXTENDED-XER encoding when an
XER encoding instruction is applied.
Annex A is informative and contains examples of BASIC-XER and CXER encodings.
Annex B is informative and contains a description of the partial XML content that is produced when constructions such
as sequence and sequence-of have their surrounding tags removed, together with restrictions on EXTENDED-XER
specifications that enable easy determination of the ASN.1 component that an XML element is associated with.
Annex C is informative and contains examples of XER encoding instructions and of the corresponding
EXTENDED-XER encodings.
© ISO/IEC 2021 – All rights reserved
Rec. ITU-T X.693 (02/2021) vii

INTERNATIONAL STANDARD
ITU-T RECOMMENDATION
Information technology – ASN.1 encoding rules:
XML Encoding Rules (XER)
1 Scope
This Recommendation | International Standard specifies a set of basic XML Encoding Rules (BASIC-XER) that may be
used to derive a transfer syntax for values of types defined in Rec. ITU-T X.680 | ISO/IEC 8824-1 and Rec. ITU-T
X.681 | ISO/IEC 8824-2. This Recommendation | International Standard also specifies a set of Canonical XML
Encoding Rules (CXER) which provide constraints on the basic XML Encoding Rules and produce a unique encoding
for any given ASN.1 value. This Recommendation | International Standard further specifies a set of extended XML
Encoding Rules (EXTENDED-XER) which adds further encoders options, and also allows the ASN.1 specifier to vary
the encoding that would be produced by BASIC-XER. It is implicit in the specification of these encoding rules that
they are also used for decoding.
The encoding rules specified in this Recommendation | International Standard:
– are used at the time of communication;
– are intended for use in circumstances where displaying of values and/or processing them using
commonly available XML tools (such as browsers) is the major concern in the choice of encoding rules;
– allow the extension of an abstract syntax by addition of extra values for all forms of extensibility
described in Rec. ITU-T X.680 | ISO/IEC 8824-1.
This Recommendation | International Standard also specifies the syntax and semantics of XER encoding instructions,
and the rules for their assignment and combination. XER encoding instructions can be used to control the EXTENDED-
XER encoding for specific ASN.1 types.
2 Normative references
The following Recommendations and International Standards contain provisions which, through reference in this text,
constitute provisions of this Recommendation | International Standard. At the time of publication, the editions indicated
were valid. All Recommendations and Standards are subject to revision, and parties to agreements based on this
Recommendation | International Standard are encouraged to investigate the possibility of applying the most recent
edition of the Recommendations and Standards listed below. Members of IEC and ISO maintain registers of currently
valid International Standards. The Telecommunication Standardization Bureau of the ITU maintains a list of currently
valid ITU-T Recommendations.
NOTE – This Recommendation | International Standard is based on ISO/IEC 10646:2003 and the Unicode standard version
3.2.0:2002. It cannot be applied using later versions of these two standards.
2.1 Identical Recommendations | International Standards
– Recommendation ITU-T X.680 (2021) | ISO/IEC 8824-1:2021, Information technology – Abstract
Syntax Notation One (ASN.1): Specification of basic notation.
– Recommendation ITU-T X.681 (2021) | ISO/IEC 8824-2:2021, Information technology – Abstract
Syntax Notation One (ASN.1): Information object specification.
– Recommendation ITU-T X.682 (2021) | ISO/IEC 8824-3:2021, Information technology – Abstract
Syntax Notation One (ASN.1): Constraint specification.
– Recommendation ITU-T X.683 (2021) | ISO/IEC 8824-4:2021, Information technology – Abstract
Syntax Notation One (ASN.1): Parameterization of ASN.1 specifications.
– Recommendation ITU-T X.690 (2021) | ISO/IEC 8825-1:2021, Information technology – ASN.1
encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER).
Rec. ITU-T X.693 (02/2021) 1
© ISO/IEC 2021 – All rights reserved

– Recommendation ITU-T X.691 (2021) | ISO/IEC 8825-2:2021, Information technology – ASN.1
encoding rules: Specification of Packed Encoding Rules (PER).
– Recommendation ITU-T X.692 (2021) | ISO/IEC 8825-3:2021, Information technology – ASN.1
encoding rules: Specification of Encoding Control Notation (ECN).
– Recommendation ITU-T X.891 (2005) | ISO/IEC 24824-1:2007, Information technology – Generic
applications of ASN.1: Fast infoset.
NOTE – The references above shall be interpreted as references to the identified Recommendations | International Standards
together with all their published amendments and technical corrigenda.
2.2 Additional references
– IETF RFC 2045 (1996), Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet
Message Bodies.
– IETF RFC 2141 (1997), URN Syntax.
– IETF RFC 2396 (1998), Uniform Resource Identifiers (URI): Generic Syntax.
– IETF RFC 3061 (2001), A URN Namespace of Object Identifiers.
– ISO/IEC 10646:2003, Information technology – Universal Multiple-Octet Coded Character Set (UCS).
– The Unicode Standard, Version 3.2.0, The Unicode Consortium. (Reading, MA, Addison-Wesley)
NOTE 1 – The graphics characters (and their encodings) defined by the above reference are identical to those
defined by ISO/IEC 10646, but the above reference is included because it also specifies the names of control
characters.
– W3C XML 1.0:2008, Extensible Markup Language (XML) 1.0 (Fifth Edition), W3C Recommendation,
Copyright ©2008 W3C® (MIT, ERCIM, Keio), https://www.w3.org/TR/2008/REC-xml-20081126/ .
– W3C XML Namespaces:1999, Namespaces in XML, W3C Recommendation, Copyright © 1999 W3C
(MIT, INRIA, Keio ), http://www.w3.org/TR/1999/REC-xml-names-19990114.
NOTE – The reference to a document within this Recommendation | International Standard does not give it, as a stand-alone
document, the status of a Recommendation or International Standard.
3 Definitions
For the purposes of this Recommendation | International Standard, the definitions of Rec. ITU-T X.680 | ISO/IEC
8824-1 and the following definitions apply.
3.1 ASN.1 Basic Encoding Rules (BER)
This Recommendation | International Standard makes use of the following terms defined in Rec. ITU-T X.690 |
ISO/IEC 8825-1:
a) data value;
b) dynamic conformance;
c) encoding (of a data value);
d) receiver;
e) sender;
f) static conformance.
3.2 Additional definitions
For the purposes of this Recommendation | International Standard, the following definitions apply.
3.2.1 ASN.1 schema:  The definition of the content and structure of data using an ASN.1 type definition.
NOTE – This enables encoding rules to produce binary encodings of the values of an ASN.1 type, or encodings using XML.
3.2.2 associated empty-element tag: The XML empty-element tag that can replace an associated preceding tag and
an associated following tag, when present.
NOTE – Some encoding instructions remove the associated tags of an "XMLValue".
3.2.3 associated encoding instructions (for a type): A set of XER encoding instructions associated with a type.
2 Rec. ITU-T X.693 (02/2021)
© ISO/IEC 2021 – All rights reserved

3.2.4 associated following tag: The XML end-tag following the "XMLValue" of a type in the absence of encoding
instructions that remove the associated tags.
3.2.5 associated preceding tag: The XML start-tag preceding the "XMLValue" of a type in the absence of
encoding instructions that remove the associated tags.
3.2.6 canonical encoding: A complete encoding of an abstract value obtained by the application of encoding rules
that have no implementation-dependent options. Such rules result in the definition of a 1-1 mapping between
unambiguous and unique encodings and values in the abstract syntax.
3.2.7 canonical valid XML document (for an ASN.1 schema): An XML document which is well-formed (see
W3C XML 1.0) and whose content conforms to the CXER specification for the encoding of an ASN.1 type specified by
an ASN.1 schema.
3.2.8 character-encodable type: An ASN.1 type to which an ATTRIBUTE encoding instruction can be applied
(see 20.2.1).
3.2.9 control namespace: A namespace that is used to identify attributes that perform functions or carry values that
control an EXTENDED-XER encoding.
NOTE 1 – An example would be a type identification attribute. The control namespace defaults to the ASN.1 namespace
specified in 16.9, but can be changed by the GLOBAL-DEFAULTS encoding instruction.
NOTE 2 – The control namespace may also contain names for attributes that may be present, but which are ignored by
EXTENDED-XER decoders (see 10.2.10). An example of such an attribute name could be schemaLocation.
3.2.10 enclosed (ASN.1) type: An ASN.1 type whose "XMLValue" in a BASIC-XER encoding is enclosed directly
within the "XMLValue" of an ASN.1 type (the enclosing type).
NOTE – All types in a BASIC-XER or EXTENDED-XER encoding are enclosed types unless they are used as the root type (see
10.3.1 b) in an encoding.
3.2.11 enclosing element (of an ASN.1 type): An "ExtendedXMLTypedValue", "ExtendedXMLChoiceValue",
"ExtendedXMLNamedValue" or "ExtendedXMLDelimitedItem" that has as its "ExtendedXMLValue" the
"ExtendedXMLValue" encoding of the type (see 17.1, 17.5, 17.6 and 17.7).
3.2.12 enclosing type (of an ASN.1 type): An ASN.1 type whose "XMLValue" in a BASIC-XER encoding directly
encloses the "XMLValue" of an ASN.1 type (an enclosed type).
NOTE – The enclosing type can be a sequence type, a set type, a choice type, a sequence-of type, a set-of type, an open type, or
an octetstring or bitstring type (with a CONTAINING and without an ENCODED BY).
3.2.13 final encoding instructions (for a type): The set of XER encoding instructions associated with a type as a
result of the complete ASN.1 specification, and which are applied in producing encodings of that type.
3.2.14 inherited encoding instructions: XER encoding instructions that are associa
...