The is designed to enable associations between codes in another code system and , where the following constraints apply:
- Some of the cannot be mapped to an individual SNOMED CT .
- Licensing conditions (or other considerations) prevent addition of new SNOMED CT concepts to represent the same meaning as the .
- The can be logically defined using the to represent the same meaning or a similar though less specific meaning .
- Other requirements similar for those applicable to mapping may also apply including:
- An indication of the degree of correlation between the and the SNOMED CT expression.
- An indication of whether the was created before any single concept representation of that meaning in SNOMED CT or whether the single concept representation in SNOMED CT predated the creation of the association.
The general approach to the above requirements is to associate each of the with a representation of the same logic based definition as would have been applied to a SNOMED CT concept with that meaning. However, since the are not identified by an , the logical definition cannot be represented using . There are two potential approaches to this, one would be to use a general purpose description logic language (e.g. ) and the other is to use a to represent each definition. The is designed to support the expression-based approach.
Code to expression type reference set - Data Structure