Twelve Ranks Of Roman Military Officers

The Roman Empire and the Indian Ocean: The Ancient World Economy & The Kingdoms of Africa, Arabia & India, Raoul McLaughlin, c. 2014, 2018.

From this source first. 

Senators: top social class; generally hereditary

• General officer: Legatus Augusti proparetore: the highest rank an officer could achieve; governor of a province; only 40 provinces at the time, so very few attained this position
• Senior officer: Legatus legionis: commanded a legion, a force of around 5,000, divided into 10 cohorts; he was a senator, usually in his early 30s
• Junior officer: Tribunus laticlavius: second-in-command of a legion; the most junior officer role of the senatorial class; it was here that most sons of senators began their military service

Equestrians: next social class down from the senators; ~ medieval knights

• Praefecti

• Praefecti of a cavalry unit:
• Tribunis anticlavius: note similarity to junior officer of the Senate class
• Praefecti of an auxiliary infantry cohort (ten cohorts in a legion, so about 500 men); coanding an auxiliary unit did not have the same status as commanding a unit in the legions; auxiliaries were recruited from the provinces rather then Roman citizens; seen as lower quality troops;

• Tribunus angusticlavii: the Tribunus angusticlavii was between the two praefecti positions; staff officers; five of them

TDY: Vexillation: a detachment of soldiers; on campaign elsewhere; to carry out a specific project; led by Tribuni angusticlavii; this would give them a chance to lead not just auxiliaries but legionaries

Senior NCO: Praefectus castrorum: third in command; the camp prefect; usually an experienced soldier who had been in the army for most of his adult life; required technical knowledge of how the legion worked

Leadership of a legion:

• an inexperienced officer from the upper class
• a grizzled veteran of great experience but low social standing

A squadron commander: Centurion: the lowest position an equestion might hold; also the highest an ordinary soldier could achieve; they commanded centuries (usually about 80 men -- 100 = a century); some men started their careers as centurions because they were wealthy or because they were equestrian who could not fine a praefecti role; others were average non-royal, non-wealth men, in junior rank, working themselves up the career ladder; this was a path to wealth, status, maybe even a role in local administration

Primi orines: most senior centurions

The century was the unit that men most identified with (a squadron) but a cohort of six centuries was the basic battlefield unit of a legion.

Pilus prior: it is unknown who led a chorot in battle but it may have been a pilus prior, the commander of a senior century in the legion

• Optio: the centurion's second-in-command,

Principales: three posts

• Optio: see above; second-in-command of a squadron; second to the centurion
• Signifier: carried the unit's (century's) eagle standard; a very prestigious post
• First sergeant: Tesserarius: the guard commander for the century