 They existed therefore as a halfway house between "numeri", who were encouraged to be Romanised, and the "foederati", who raised officers from their own ranks and were almost entirely self-dependent.A distinction between frontier guard troops and more mobile reserve forces had emerged with the use of certain troops to permanently man frontiers such as Hadrian's Wall in "Britannia" in the 2nd century AD.The competing demands of manned frontiers and strategic reserve forces had led to the division of the military into four types of troops by the early 4th century