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.