I work with AD data, and it occurred to me I don't actually know what 'Primary User' means.
(I don't work directly with AD, so it's possible I'm asking the wrong question or need to clarify - do let me know if that's the case).
Edit: it seems, as per Cheekaleak's answer, that I'm looking at an organisation-specific attribute. Flagging for close as this question will not be useful to others.