SNOMED CT subtype Relationships provide a logical semantic hierarchy. Often it is possible to view parts of the terminology and select particular Concepts by navigating through this subtype hierarchy. However, there are many situations in which the pure subtype hierarchy does not provide an ideal route for navigating the hierarchy (see 5.5.1. Using the Subtype Hierarchy).
Navigation hierarchies can be used to drive some types of structured data entry. Navigation hierarchies can order data in sensible ways by priority, or by some readily understood convention (e.g. "cranial nerve order" or "pharmacy products in the order of strength"). Navigation hierarchies can be used for diverse purposes (e.g. "topics related to diabetes").
Navigation links are used to provide an alternative route through parts of the terminology. A navigation link can link any two Concepts together to identify a useful route for navigation. Each of the navigation links is directional, linking a navigational parent Concept to a more refined navigational child Concept. However, unlike the subtype relationship the presence or absence of a navigation link neither adds to nor subtracts from the definition of either of the Concepts that it links.
Some Concepts may exist only to provide nodes in a navigation hierarchy. These Concepts are subtypes of | Navigational Concept| and play no part in the semantic definitions of any other Concept.
Navigational hierarchies are represented as Reference Sets, which are available in the International Release of SNOMED CT or they can be created from scratch to meet specific user needs. Navigational hierarchies created from scratch do not have to represent subtypes or logical relationships unless it is required. (see 6.2.5 Implementing Navigation Hierarchies).