For those that don't know, you're generally either a commissioned officer (with ranks from 0-1 and up) or enlisted (E-1 to E-9). Some branches have warrant officers too but let's ignore that.
So if you join as an enlisted you start off as a private in the Army (it's called something else in different branches). By the time you finish bootcamp you're an E-2 private, possibly an E-3 (Private First Class). If you're not an E-3 it's automatic promotion after ~6 months assumpting you don't have any red flags AFAIK.
By the time you make it to E-4 (Corporal in the Army) you kinda know how things work BUT you're also in the last rank before you're in a leadership position. The next position (E-5, Sergeant in the Army) is a noncomissioned officer ("NCO"). Some people want to avoid that so they kinda hang around E-4 far longer than they should and they build up a body of knowledge on how to get things done. Or they may have been a higher rank and get busted down from an Article 15 (or NJP or whatever the specific branch calls it).
Requisitions can be a huge issue in the military, evne for simple things like office supplies. So you may find that E-4s can "acquire" needed supplies from other units. NCOs, Staff NCOs and command tend to be aware of it but will ignore it because it kinda needs to happen. And those E-4s are called the "E4 Mafia".
This, I believe, is the kind of "rule breaker" this post is referring to.