Subinfeudation is a modern coinage. Although the concept is implicit in the tenurial structure of Domesday Book, the word itself does not occur there.
After the Norman Conquest, almost all land held by laymen in 1066 was redistributed by the Conqueror. Those to whom he granted large numbers of estates - his tenants-in-chief - were expected to supply military service. The tenants-in-chief in turn granted their own followers some of the manors they had received from the Crown, in return for service which was probably often of a military nature. This process was known as subinfeudation. The tenant of a subinfeudated manor might, in his turn, subinfeudate land to a subtenant, creating a further rung on the tenurial ladder, which could be extended downwards almost indefinitely.
The creation of new dependent tenures continued until the late thirteenth century. Whether, at this stage, these subinfeudated holdings constituted fees owing defined amounts of knight service is still an open question; Domesday Book is unforthcoming on the subject.
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); Susan Reynolds, Fiefs and vassals: the medieval evidence reinterpreted (1994); and T.S. Purser, 'The origins of English feudalism? An episcopal land-grant revisited', Historical Research, vol. 72 (2000), pages 80-92.