ISO2709-based formats (USMARC, etc.). Only records with a two-level structure (corresponding to fields and subfields) can be directly mapped to ISO2709. For records with a different structuring (eg., GILS), the representation in a structure like USMARC involves a schema-mapping (see the Section called The Schema Mapping (.map) Files), to an "implied" USMARC schema (implied, because there is no formal schema which specifies the use of the USMARC fields outside of ISO2709). The resultant, two-level record is then mapped directly from the internal representation to ISO2709. See the GILS schema definition files for a detailed example of this approach.
Explain. This representation is only available for records belonging to the Explain schema.
Summary. This ASN-1 based structure is only available for records belonging to the Summary schema - or schema which provide a mapping to this schema (see the description of the schema mapping facility above).
SOIF. Support for this syntax is experimental, and is currently keyed to a private Index Data OID (1.2.840.10003.5.1000.81.2). All abstract syntaxes can be mapped to the SOIF format, although nested elements are represented by concatenation of the tag names at each level.