So, in prescribing something you effectively make it invalid? Mandy, I think the meaning of legal prescription might be clearer if we review its evolution:
To write before -> to lay down authoritatively as a guide, direction or rule -> to impose as a peremptory order; to direct, define, mark out (= old English use = to outlaw someone) -> to assert a right or title to the enjoyment of a thing, on the ground of having previously had an uninterrupted and immemorial enjoyment of it, hence prescribing any claim against that right or title.
The most common current use of prescription in US law is in the property law concept of
prescriptive easement. A presciptive easement is a right to use another's property not inconsistently with the owner's rights, acquired through an open, notorious, adverse and continuous use for a statutory period (IIRC, 15 years in Michigan). For links to some prescriptive easement cases, see
http://courtofappeals.mijud.net/Digest/topiclists/Property_pg3.htmPersonally, I find the professiocentric term "set-off" most annoying, given that the rest of the world uses "offset."
