The Expression Constraint Language is a formal syntax for representing SNOMED CT expression constraints.
Expression constraints are computable rules used to define a bounded sets of clinical meanings represented by either precoordinated or postcoordinated expressions.
Expression constraints can be used to restrict the valid values for a data element in an EHR, as the intensional definition of a concept-based reference set, as a machine processable query that identifies a set of matching expressions, or as a constraint that restricts the range of an attribute defined in the SNOMED CT concept model.
The has full details on ECL, including use cases, requirements, logical model, syntax specification, examples, and implementation considerations.