This is one of InLOC's direct properties.

The primary structure refers to the LOCstructure where the primarily definitive LOCassociations can be found for the LOCdefinition with that primaryStructure.

In straightforward cases, where all the LOCdefinitions are defined afresh within a LOCstructure, primaryStructure is not needed. Its role is to point to one main place – either (most likely) a clear "home" for a LOCdefinition, or in the unlikely event of it not having a single home, a place where its context is sufficiently rich and clear to explain the detail of what the LOC definition means.