This is one of InLOC's direct properties.

Bearing in mind that several identifiers can refer to the same LOCstructure or LOCdefinition, extra identifiers may be used to allow cross reference with documentation outside the current InLOC document, The extra identifiers occur only as direct properties of LOCdefinition and LOCstructure, and must not be used elsewhere in the same InLOC document.

The main identifier of a LOCdefinition or LOCstructure within an InLOC document must be the id, and it is this id that is used for reference within the InLOC document, not any extraID.