What is “a fief” usually exchanged for?

The story behind the answer

In the feudal system, a fief—often a grant of land, revenues, or rights—was given by a lord to a vassal in return for armed support. The vassal swore homage and fealty and was obliged to ride to war, provide a set number of days of knight service (commonly around forty per year), and help defend the lord’s castles.

This arrangement bound medieval society together: lords gained loyal fighters without keeping standing armies, while vassals gained resources to equip themselves and their retinues. Other duties included giving counsel in the lord’s court and providing “aid” for ransoms or dowries; sometimes a cash payment (scutage) could substitute for campaigning. Labor dues were typical of peasants under manorialism, and vassals held land by tenure rather than full ownership.