Status Updates From La grande strategia dell'Im...

Clearly, since the Roman Army was no longer an undifferentiated force apt to fight anywhere, regional patterns of deployment had become useful: dromedary troops for the desert, mounted archers for open frontiers such as those of Dacia & above all the Euphrates, light spearmen for mountain country [with echoes of Velites & Macedonian peltasts?] and so on.

THE ROMAN EMPIRE HAD NO REAL INNER LINES.
The "donut" shame around the Mediterranean in an age were sea travel was unpredictably slow rendered the concept of a strategic reserve useless, compared to what Luttwak calls a "regional defence policy" : hold with local forces until relieved by Legions X day marches away.

The army under Tiberius served as a mobile striking force estimated at 150.000 legionaries plus an equal number of auxiliaries. For practical purposes their deployment was that of a field army on potential hot spots between recently pacified & unconquered lands.