The Expression Constraint Language (ECL) is used to define the reference set members based on their supertypes, values of defining relationships, and membership in other reference sets etc. To read more about the ECL, please refer to the Expression Constraint Language Specification and Guide 

Reference Set members are selected individually. I.e. when an author browses the SNOMED CT content, explores and evaluates selected candidate members and determines what components to be included.

Manual Enumeration

Intensional Definition

The reference set is refined by excluding specific parts of the initial set.

The initial set of members is expanded by adding components which have a logical association to the components referenced in the initial set. 

Member Inclusion

Member Exclusion

Member inclusion or exclusion can be done by manual enumeration or by intentionally defining the members to be included/-excluded. A combination of both approaches may also apply.

Reference set members can also be identified using a combination of both manual enumeration and intensional definition.