Late Roman Unit Sizes

[Nathan Ross]

I have yet to find a scholar who supports the idea that – after all – the auxiliaries of the Principate military system were still in existence after Caracalla extended the Roman citizenship to every citizen within the Empire.

Surely they did still exist - some of them appear in the ND, after all, occupying the same stations they'd held since the principate - but they were no longer called 'auxiliary' cohorts?

A rescript of Diocletian (C.J. 10.55(54).3) informs a certain Philopater that he is not eligible for the discharge exemptions due to veterans of legions or vexillations (of cavalry, presumably), since he 'served in a cohort'. So the former auxiliary units were still classed as lower grade troops, even if they were citizens.

While Ammianus mostly uses the word auxiliary to refer to the auxilia palatina, he also mentions auxiliary troops coming from various foreign peoples - more an adjective than a noun, in that case. Neither usage is the same as the old principiate auxiliaries though. The new constitution of 212, as you say, seems a good guess for the change of title.