An incredibly useful pool of experience, kept on as a Civil Serpent after retirement to do jobs that don't need a young and thrusting career minded individual, or MATT2 fitness standards, to do but we would still like done by somebody with recent service experience and a solid understanding of the way the Army works. Often found as Officers' Mess Secretary (and 2nd-hand clothing maestro), in the duller jobs in Training Establishments, or running weird bits of the countryside that the main body of the Army haven't seen since 1944.
Crusty old farts who couldn't hack it in the civvy world, even as a school bursar, after they were finally ejected from the seats by the Mess bar they had been occupying since the Regiment returned from Korea. Found some sinecure where they can sit reading the Torygraph or the Mail until the bar opens, from where they can concentrate on their true endeavour - complaining about everything, especially subalterns. The truly evil ones, especially some Colonels who never actually commanded, will continue to wear uniform and forget to buy the rank slides with the "R" on them.