Setback as I interpret the document below, is the buildable part of the lot. In TV, I was told by Steve Mansfield (as an example of setback) that the patio walls being built in new sections by his company must be within the setback.
setback legal definition of setback. setback synonyms by the Free Online Law Dictionary.
From the above site searching for "easement"....this definition....
easement n. the right to use the real property of another for a specific purpose. The easement is itself a real property interest, but legal title to the underlying land is retained by the original owner for all other purposes. Typical easements are for access to another property, (redundantly often stated "access and egress," since entry and exit are over the same path), for utility or sewer lines both under and above ground, use of spring water, entry to make repairs on a fence or slide area, drive cattle across, and other uses. Easements can be created by a deed to be recorded just like any real property interest.