I'd like a bit of clarification on Rex's role within the 501st

Captain Rex was a Clone Captain in the 501st, and he served primarily under Skywalker.I'm assuming that just because he served primarily under skywalker and was his second in command, doesn't necessarily make him commander of the 501st legion itself.I'm a bit fuzzy on Clone Ranking Systems, but I believe the Clone Commander was the highest rank a clone could achieve during the Clone Wars (Commander Cody for example).My question is this: Was Rex just the leader of Torrent Company, within the 501st? Or was he the commander of the 501st in all but name. If he wasn't, I'm assuming that either Anakin was always just the presumed leader of the 501st so he would hold the title of Commander/General of the legion, or there was some other unknown Clone Commander before Appo (because Appo was a sergeant at the time of the umbaran arc) that was technically in charge. Thanks guys. via /r/StarWars http://ift.tt/2qqBihz