After Augustus reforms the rather heterogeneous collection of auxiliary units serving Rome was completely reorganized, given regular status and trained to the same standards of discipline as the legions
The men of the Auxilia were freeborn non-citizens living on the periphery or the Empire with a heavy preponderance of Gauls, Thracians and Germans and were rewarded with the Roman citizenship on honourable discharge from the Army
The higher organization of auxiliary infantry was the cohors peditata that came in two flavours: quingenaria, modelled after the typical legionary cohors and the most common by far, and milliaria, modelled after the legion’s cohors prima
The cohors peditata quingenaria was under command of a prefect and consisted of six centuriae of 10 contubernium totalling some 480 men. Around the second half of the 1st century appeared a new kind of unit named cohors peditata milliaria which included ten centuriae instead of just six, was under command of a tribune and had a theoretical strength of 800 men.
Besides these pure infantry units auxiliary were also organized into pure cavalry (ala) and mixed (cohors equitata) formations.