eli5 Where do the numbers in military units come from?

Question

I always see, for example, military units, like the 126th division or whatever, but I never see any mention of divisions 1-125, or 127 and beyond, etc. The numbers always seem so arbitrary, but there has to be a reason they’re used to name the units. Can someone please explain how this works to me? Thanks.

In: Other

Answer 1

Not sure how the Army does it, but in the Air Force, if it’s a 3 digit unit, a 1 in the hundreds place signifies it’s a National Guard unit, a 9 signifies a Reserve unit, 7 and 8 signifies it’s an auxiliary unit and has a sister unit at the same base.