Definitions of prescription
a plan to help achieve a particular result
What is your company's prescription for success?
an instruction written by a doctor for a patient to be given a particular drug, or the piece of paper on which the instruction is written
I'm sorry, that's only available on prescription.
the establishment of a claim to some title, right or privilege on the basis of a long or indefinite period of uninterrupted use of something; a claim on the basis that a particular thing (eg a custom) has always been done and accepted by law; the action of making such a claim
The common law idea of prescription relates to rights enjoyed through custom rather than written law.