Subinfeudation was the feudal custom of vassals holding vassals of their own. In the multiple layers of feudal obligation in medieval Europe such subinfeudation was common, with lords owing fealty to a greater lord, and receiving fealty from lesser ones.
This is not to say the the situation was always -- or even often -- that simple. In many cases, a medieval lord would owe leal service to multiple overlords, and could find himself in a sticky situation -- in a worst case owing conflicting service to two overlords at odds with one another.
Christine de Pizan addresses such situations in The Book of Deeds of Arms and of Chivalry with the advice that a conflicted vassal still owed service to both suzerains: he would be required to furnish troops for to each of his overlords, personally commanding the contingent sent to the lord to whom he owed "higher" service and a trusted lieutenant (preferably a close kinsman or son) commanding the other; his overlords in turn were obliged to ensure that their vassal's troops never faced each other in battle. (It is unclear whether this theoretical solution was ever put into practice.)
Another problematic aspect of subinfeudation was the obligation of the lord to the vassal and whether or not that obligation extended to the vassals of his vassal, a debate which had serious implications in period.