SNOMED CT represents relationships between concepts that are necessarily (i.e. always) true. However, other relationships between concepts may exist in specific situations or use cases. An 5.4. Association Reference Set can be used to represent these additional relationships, which are not necessarily true, but which are needed for a specific purpose. Examples include:
5.4. Association Reference Set can be used to constrain (or guide) data entry into fields, where the value is dependent on (or has some type of association with) the value of another field. While other technical solutions are possible, the 5.2.5 Association Reference Set provides a standardized way of representing and distributing the associations required to support this functionality. The figure below illustrates how an 5.2.5 Association Reference Set could be used for this purpose.
Figure 18.104.22.168-1: Using associations to define dependencies between fields