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SNOMED CT contains hundreds of thousands of Concepts and this rich resource is greatly expanded by use of postcoordinated expressions. In any given situation the range of Concepts or expressions that are useful, relevant and meaningful is much more limited. This gives rise to a requirement to represent constraints on the content or a particular field in a way that can be interpreted and applied by application software.

The simplest constraint requirements can be met by specifying the list of valid codes. This requirement is addressed by subsets specified using the Reference Set mechanism. In some cases, it is useful to express the range of possible values 'intensionally' by specifying rules rather than by listing every member of the set (e.g. to include all concepts that are subtypes of a specified concept ).

The use of postcoordinated expressions adds further dimensions to the requirement for constraints. It may be necessary to specify whether all postcoordinated refinements of concept are permitted or whether some types of refinement are prohibited or required. It may also be necessary to specify whether a postcoordinated expression that is equivalent to a permitted value is itself permitted.

Requirements for representing expression constraints are closely related to the requirements for representing query predicates in queries.