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When a new version of SNOMED CT is released this may include changes to the content of the terminology. Components may have been added, inactivated or changed as described in 11.4 Versioning. As part of the terminology maintenance process, it may be appropriate to evaluate both the Representing Reasons for Component Inactivation and to determine appropriate alternatives.

The International Edition of SNOMED CT distributes a set of reference sets that record the reason that each inactive component was inactivated. These are referred to as "historical association" reference sets (see 4.2.3 Historical Association Reference Sets for more details). There is one historical association reference set for each type of historical association as shown in the table below.

Association reference set types in the International Release of SNOMED CT

Association reference set	Descriptions
	Applies to a that is ambiguous. The targetComponent is an active that represents one of the possible meanings of the inactive . Multiple rows are used to refer to each of the possible meanings of the ambiguous
	Applies to a that has been moved to (or are pending a move to) another namespace. The targetComponent identifies the target namespace (not the new ).
	Applies to a that has been moved to this namespace from another namespace. The targetComponent identifies the original in its previous namespace.
	Applies to an erroneous, obsolete and other for which there is a single active replacement. The targetComponent identifies the active that replaces this
	Applies to a that is a duplicate. The targetComponent identifies the active that this duplicates.
	Links an inactive classification such as "not otherwise specified" or "otherwise specified" with the active that was formerly its most proximal supertype.
	(not used currently)
	Links an inactive classification derived from ICD-9 Chapter XVI "Symptoms signs and ill-defined conditions" with the most similar active
	Applies to an inactive which is inappropriate to the it is directly linked to but instead should refer to the referenced by the targetComponent.

The following table holds example entries for the reference set. With this reference set is possible to automatically identify that the inactive concept should be replaced with the concept . This type of reference set is particularly useful for ensuring consistent use of SNOMED CT over time. The reference set mechanism here provides an easy and standardized way of managing changes to coding or documentation practice over time.

Sample content from 900000000000526001 | REPLACED BY association reference set |

referencedComponentId	referencedComponentId_term	targetComponentId	targetComponentId_term
100005	SNOMED RT Concept	138875005	SNOMED CT Concept
212002	Salmonella III arizonae 53:k:z	398450001	Salmonella IIIb 53:k:z
225005	Special care of patient with contagious disease	133895001	Care of patient with infectious disease
244003	Evans and Lloyd-Thomas syndrome	66659007	Normal variation in position
278009	Epidural injection of neurolytic substance, lumbar	17753007	Epidural injection of neurolytic solution, lumbar
558000	Other disorder of the neurohypophysis, NEC	72442006	Disorder of posterior pituitary
659001	Peptostreptococcus anaerobius	413524006	Anaerococcus tretradius
696005	Chronobiologic disorder	387605007	Abnormal chronobiologic state
700002	Salmonella III arizonae 50:z4,z23,z32:--	404619004	Salmonella IIIa 50:z4,z23,z32:-
822000	Salmonella arizonae 53:z4,z23:--	13998005	Salmonella IV 53:z4,z23:--