Via Starwars.com:

From the outset of organizing the 501st Squad in 1999 into a meaningful structure of units around the world, it was a question of how to best keep the feel of a fictional military unit while creating practical regional chapters. Luckily I had a consultant in my old college buddy Alan Isom, TK820. One of the original troopers there in 1998 at the first Dragon Con meeting, he’s had twenty five years in the National Guard. I asked him, “How do we do this as a tribute to the military without posing as a paramilitary group or insulting the armed services with terminology that mimicked theirs?”

Alan was the perfect person to ask. On top of from his military experience, he and I share a love of military history. So we agreed that anachronistic terms would work and still capture the feel of something a “long time ago.” If the Legion was an Imperial unit occupying earth, as many of our early advertisements hinted, then what would a stationary unit covering a large area be called? Garrison made sense, the Romans stationed Garrisons all over their conquered lands. But what about a small unit in a remote land with only a handful of soldiers? Outpost seemed to fit best. And later, when our membership covered every incarnation of Imperial several times over, I thought it best to organize similar costumes into themed groups. Well, Alan pointed out, any specialized group sent on a mission could be called a Detachment.

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