general question - what leads to the numbering of units?

[raketenjagdpanzer]

What is the decision process that goes in to that? 101st airborne division...well, there weren't 100 airborne divisions that preceded it, were there?

394th Tank Guards, 5th armored (actually we did used to have 5 armored...excuse me, ANY armored divisions), 344th TFW -

what's the pattern here? Or is there none? Does someone just come up with a unit composition and stick a number to it, whatever sounds good that day? "Hey, have we used 199th? No? 199th it is."