https://www.hl7.org/fhir/valueset.html#compositions

The ValueSet gives details on how to expand a value set definition, including cross-referencing to things like the VSD CLD, but omits to point back at the CodeSystem filters section (https://www.hl7.org/fhir/codesystem.html#filters) which contains essential specific details on how, for example, the '=' operator should behave with respect to certain datatypes and values.