It's an issue that came up recently, when someone returning from NFTC (NATO Flying Training Canada, where up until recently the RAF has sent 20 fast jet studes a year to conduct the Tac/Weapons course otherwise done at RAF Valley on 19 Sqn) discovered that all the studes since, like, forever! had been entitled to claim IE (incidental expenses) to cover living away from home, but no one had been told this, hence a whole bunch of decently sized back-claims have arrived in the system....