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Code systems may be presented hierarchically, using nested concept elements, where the hierarchy has a defined meaning specified in hierarchyMeaning. The code system hierarchy is a single tree, where concepts have only one parent.

Some code systems define concepts that have multiple parents. If a code system has concepts like this (typically, concepts that are subsumed by more than one other concept), the code system SHOULD NOT be represented using the hierarchy structure in the Code System resource, and the relationships between concepts should be defined using properties. If the hierarchy is used, implementers SHALL use a property to indicate additional parents that are not represented in the structural (nested concepts) hierarchy.

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Some code systems define multiple hierarchies resulting in concepts with multiple parents.