SNOMED CT concepts are related to one another within a subtype hierarchy. At the top of this hierarchy are general concepts referred to as top-level concepts. Clinically relevant concepts in SNOMED CT fall under one of the hierarchies shown in
Clinically Relevant SNOMED CT Hierarchies
SNOMED CT concepts are also related to one another by defining relationships which represent characteristics of the meaning of a concept. Each of these relationships represents the value of an attribute. SNOMED CT currently uses more than fifty defining attributes, each of which is identified by a concept.