Was it based entirely on time-in? Subjective observations by their immediate supervisors? Or was it like a modern, Western army and based on mandated 'courses' and qualifications?
If someone can answer this question regarding Republic armies, I'm interested in those as well; I just didn't want my question to be super-vague.
Thanks in advance!
According to G. R. Watson in The Roman Soldier, promotion for the common soldier was actually pretty rare. If you were recruited as a legionary, you tended to stay one. Centurions and other field officers were recruited into their ranks, largely on the basis of having skills (such as reading and writing) which were required in performing some of the administrative work that came with the position.