The relief was a payment offered by an heir to an overlord for the right to succeed to an estate. As such, it could be confused with the heriot, a death duty rather than an inheritance tax. Domesday Book does confuse the two on occasions, as did other contemporary sources. In part, this is due to an inherent ambiguity between a death duty and an inheritance tax, both being succession payments in fact if not in name or origin.
After the Conquest, in fact, the two were fused. Thereafter, succession dues paid by peasantry were generally known as heriots, those by the aristocracy as reliefs. Reliefs also appear to have become more onerous after the Conquest. As early as the reigns of the Conqueror's two sons, they were politically charged issues, so much so that on his coronation Henry I felt obliged to renounce the arbitrary sums levied by his brother and content himself with 'just and lawful reliefs'. They remained, however, a major source of grievance down to Magna Carta.
For more detail, see Sir Fredrick Pollock and F.W. Maitland, The history of English law before the time of Edward I (second edition, 1898); A.L. Poole, Obligations of society in the XII and XIII centuries (1946); and Susan Reynolds, Fiefs and vassals: the medieval evidence reinterpreted (1994).