Tenant-in-chief is a modern coinage which, 'like many other supposedly technical terms of feudalism, seems to be a creation of medievalists rather than of the middle ages' (Reynolds, Fiefs, page 324).
By convention, tenants-in-chief are those landowners who held their lands directly from the Crown after the Conquest. In Domesday Book they are individually listed at the beginning of each county, where they are each assigned a separate section, or chapter, conventionally known as their fief. The totality of their county fiefs are conventionally called Honours. Whether, at this stage, tenancies-in-chief owed defined quotas of military service to the Crown is unclear; Domesday Book is unforthcoming on the subject.
The minor landowners who held directly from the Crown were normally grouped together in a collective fief at the end of the text for each county, though Domesday Book is not entirely consistent in its classification of major and minor tenants-in-chief. At a later date most of these lesser tenants-in-chief would be known as sergeants.
On fiefs and feudalism, see Susan Reynolds, Fiefs and vassals: the medieval evidence reinterpreted (1994); and on military quotas, see J.H. Round, Feudal England (1895); and John Gillingham, 'The introduction of knight service into England', Anglo-Norman Studies, vol. 4 (1982), pages 53-64; 181-87.