ISO/IEC 8825-4:2008 specifies a set of basic XML Encoding Rules (BASIC-XER) that can be used to derive a transfer syntax for values of types defined in ISO/IEC 8824-1 and ISO/IEC 8824-2. It 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. ISO/IEC 8825-4:2008 further specifies a set of extended XML Encoding Rules (EXTENDED-XER) which adds further encoder 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 ISO/IEC 8825-4:2008:

• 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 ISO/IEC 8824-1.

ISO/IEC 8825-4:2008 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.