Inland means 'inner estate', and was that part of the lord's lordship land which was exempt from paying tax. Normally this would be that part of a rural property farmed by the lord for his own profit, as opposed to the land of his peasants. Whether this tax emption was enjoyed by all landowners, or simply by those privileged by the Crown, is unclear and has long been debated.
The bulk of the references to inland occur in circuit 6, disproportionately in Lincolnshire. These circuit 6 references often equate inland with outliers, a distinct, if related meaning. Inland in its more typical sense was certainly more widely distributed than Domesday suggests, and probably existed in most parts of the country.
For more detail, see Paul Vinogradoff, English society in the eleventh century: essays in English medieval history (1908); F.M. Stenton, Types of manorial structure in the northern Danelaw (1910); and Rosamond Faith, The English peasantry and the growth of lordship (1997).