In the SNOMED CT Release File Specification (http://snomed.org/rfs), SNOMED International specifies a set of reference set types with their own specific properties (e.g. an attribute value type reference set). Each reference set that is developed to conform to a specified type is defined as a subtype of the associated reference set type concept (e.g. ). All reference sets of a given type are populated with members using the same data structure - with the same set of field names in the same order. SNOMED International uses these reference set type data structures (as defined in the Release File Specification) as the release file format for all reference sets of that type.

All reference set type concepts are a subtype of , and have an associated set of reference set descriptors in the . Some reference set type concepts are organised under one or more reference set groups (e.g. ), which represent a group of reference set types (often with different data structures).

In the Expression Constraint Language (v2.0+) reference set field names are used to indicate which field values to return, and to filter reference set members based on specific field criteria. The first (non-metadata) field in every reference set (in order '0') must always be 'referencedComponentId'. For reference sets, which are a subtype of an international reference set type, the additional field names defined in the SNOMED CT Release File Specification must be used. In all other cases, the additional field names may use any latin-script alphabetic character (a-z or A-Z) defined by the owner of the corresponding reference set type concept. Owners of a reference set type are encouraged to explicitly document these field names, keep them unchanged and publish a machine readable representation of these (following the format used below). In the absence of this, the column name from the corresponding RF2 file (with all whitespace removed) will be used.

The international reference set types and their corresponding list of field names to be used in ECL v2.0+ are shown in the table below (for information only). A normative, computable representation of this table is attached below the table. Please note that this file may be extended by implementers with national or local reference set types.