Does FNU mean "first name unknown" or "field not used"? And how do people with such entry on their IDs normally book a ticket (i.e., what do they put into the various fields required for the ticket)?

Apparently there was an IRS convention for handling circumstances where an individual only has one name. Do the airlines/IATA also have a convention for such circumstances and do they use "FNU" too?

... and did this apply to musicians such as Madonna and Prince who may have legally changed their name to one names?