Simply put: there is no single "system". The system of allegiances changes with time and location. Logically deducing, the darn system might even change several times in a person's lifetime, and not just when another related individual dies.
Probably the one constant in all this is the fact that as long as anyone in charge think that they can get away with abusing whatever system is in place at the time (coupled with inadequate or corrupt supervision), they will do so, for profit or simply because they can (perhaps, by the grace of whatever they worship).