Late Roman Field Armies

[Nathan Ross]

If we define a vexillatio as units detached from an exercitus provinciae to an campaigning exercitus led by the emperor or another highest grade officer, then every unit sent by a dux to a field army campaigning near to the dux' province is a vexillatio in the classic (2nd 3rd century) sense.

But surely a vexillatio in the older sense was a detachment drawn from a larger unit and operating under a vexillum, rather than their own standards (or, in the case of auxiliaries, several cohorts brigaded together under a vexillum)?

If so, the change in the use of the term might be because unit sizes had shrunk overall, meaning that entire units could be withdrawn from the frontiers without weakening the defences too gravely?

We do have the note in Ammianus about Constantius II ordering Julian to select several hundred men from some of his Gallic auxilia units and send them east, so there must still have been some mechanism for detaching smaller groups from larger ones. Seniores/iunores (again), perhaps?