First expired, first out is an inventory rotation method: when you pick an order, you take the lot that expires soonest, regardless of when it arrived. For perishable food it is the rule that keeps product from expiring on the shelf.
FEFO differs from FIFO when lots arrive out of date order. A supplier can deliver a newer lot with a shorter shelf life than stock already in the building, and FIFO would ship the longer-dated stock first and let the short-dated lot expire.
FEFO only works when every lot carries its expiry date and the pick follows it. In APFoodz the expiry date travels with every lot.