I would assume that some food could be bought or looted, but a army needs weapons (of good quality), missiles, horses, armor and soldiers to replace losses etc. I assume that all of this had to be shipped from some central location (I assume that the best weapons and such were made in Italy and other central Roman provinces), which would entail large supply lines if a army was fighting in Britain or Germania. So do we know how supplies were shipped to the legions in the field? Also, do we know how the Roman Army kept track of what supplies they needed, so that they could order replacements, and how they ordered supplies?