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Ordered reference sets can be used to specify and display a customized navigation hierarchy. Alternative hierarchical representations of SNOMED CT can support data entry by satisfying the requirements of a specific use case, and addressing some of the challenges of displaying an unordered polyhierarchy (as defined by SNOMED CT's subtype structure).

The figure below shows the way a navigation hierarchy is represented. The example reference set contains a set of concepts used to describe x-ray examinations of the upper and lower limbs. The resulting navigation hierarchy might usefully be extended to include other x-ray procedures but has been kept small for the purposes of the example. 

• Constraining the number of levels in the hierarchy and/or the number of concepts at each level.
  • Using many levels, each with a relatively small number of concepts, allows the most common options to be displayed with a higher priority.
  • Using fewer levels, each with a relatively large number of concepts can reduce the number of levels that needs to be navigated to find an appropriate concept.
  • Options that are never (or rarely) used can be excluded from a customized navigation hierarchy to limit the range of choices available.

• Ordering each concept at the same hierarchical level, to match user preferences or to facilitate faster access to more frequently used options.
• Ensuring that the navigation hierarchy is adapted to meet the requirements of a specific use case, without affecting the correctness of the subtype hierarchy (and associated logical inferences).