Late Roman Field Armies

[Nathan Ross]

they still sent detachments, and these could vary considerably in size.

This would depend on the exact era as well. Tetrarchic field armies were mainly composed of detachments drawn from the legions on the frontiers, some of which were still called vexillations into the 320s. But the word more commonly referred to a cavalry unit by this period.

The Notitia provides a sort of snapshot of the military organisation at an uncertain period (perhaps several periods combined). The field armies would be the comitatenses and palatinae, while the frontier armies (limitanei) comprised the units under the command of the dux of each province or area. The 'pseudocomitatenses' were probably former frontier units added to the field army at some point in the late 4th or early 5th century; other than that, I don't think we have any direct evidence of detachments augmenting the field army, although it almost certainly happened. The various units listed in the ND would have been assembled at different times, and with differing origins, but the process behind it all (if there was one) is still rather obscure!