Question

In exchange for land, a knight agrees to be a sort of sub-vassal to a vassal. This agreement was known as:

Answer 1

It's known as Subinfeudation

Under this agreement, the Tenants who hold a land under the King or any other superior lord could carved out a new occupant ( in this case a knight) in exchange for that occupant's Loyalty. Usually to increase the land's man power against invaders