[otrs] Difference between tn and id

Hey guys, I'm modifying OTRS fairly heavily for a large implementation. Part of this is creating tickets from our own customised interface, which we've done. The main issue is with the fields TN and ID. Does anyone know why tickets in OTRS have 2 identifiers? Is there a way to tell OTRS to only use one, or to always make them equal? Regards, Simon Adams