Rations are the primary food source for Units. When Upkeep is paid, Rations automatically pop near the Unit they are intended to feed at the beginning of the Side's Turn. Rations can be made unnecessary through the use of provisions.
Captured Units may not have Rations, because no one may be paying their upkeep. The Unit may be fed with provisions instead of Rations, but if the capturing Side paid Upkeep, the Rations may pop. In this case, Rations would pop at the beginning of the capturing Side's Turn, not the Side of the Unit.