I passed my FAA Commercial two months ago in the USA at a large and well regarded school, and have only a vague recollection of anybody using that acronym. Much as they love their acronyms over there, I think for rights of way, we simply remembered it but it was in some of the books.
That said, googling it, the interpretation I find online is...
Balloons
Gliders
Airships
Aeroplanes and Rotorcraft.
Which would match all our normal understanding anyhow.
G