The importance of the concept of assigns may be that it is the final and complete acknowledgement of the alienability of land, without any familial or seignorial restrictions. The lord's interest in controlling alienation inhered in a lord's interest in the nature of future tenants -- a discretionary right to accept or reject suitable and unsuitable tenants.
From the middle of the 12th century, clauses appeared in charters by which some lords allowed their tenants (quite often their officials) to alienate land to named individuals -- a confirmation in advance of a future alienation, but to a specified person. It seems clear that such ad hoc arrangements reflect more seignorial control over alienation than tenants' rights to alienate and it may have been a specific favour to a trusted servant/tenant. From the late 12th century, general assigns appeared embryonically in charters, a beginning of the recognition of tenants' freedom from seignorial control over alienation. By the early 13th century, a larger number of charters included general assigns, although the category did not become fairly widespread until c. 1230 - c. 1250. By that time, grants were being made to X, his heirs and assigns; the general category of assigns represented a general confirmation in advance of future gifts to unspecified persons, thus allowing an unfettered and unconditional right of alienation to the grantee.
The position in the early 13th century was recognised by Bracton (1220s/1230s) who stated that a lord must allow his tenant to sell or alienate.
Waugh has indicated that some lords reacted against this problem in the (late 12th and) early 13th century by making conditional grants, in which the nature of the assigns was restricted, excepting chief lords, Jews, or religious houses, or an oath was extracted not to alienate the land without prior authorization from the lord. Further, some lords in their grants inserted a right of pre-emption.