ISO 20022-8:2013 describes the transformation rules to generate ASN.1 abstract syntax from an ISO 20022 compliant MessageDefinition. The generated abstract syntax is for the description and validation of Messages.
The transformation rules are a transformation from Level 3 to Level 4. It is a deterministic transformation, meaning that the resulting ASN.1 is completely predictable for a given MessageDefinition. There is neither manual input to the transformation itself nor manual adjustment to the result of the transformation.
ISO 20022-8:2013 is the ASN.1 equivalent of ISO 20022-4:2013. In ISO 20022-4:2013 the abstract syntax generated is XML Schema; in ISO 20022-8:2013 it is ASN.1. In ISO 20022-4:2013 the only encoding supported is UTF-8 XML; in ISO 20022-8:2013 there are multiple encodings supported for ASN.1. These include all the standard encodings, but in addition the ability to register custom encodings in ECN.