A prescript is an official order or rule — something written down that you are meant to follow. It is a rare, formal noun. Lawyers, theologians, and old books use it; friends in a café do not. You will far more often meet prescription (a doctor’s note, or a stronger “this is the rule”) and prescribed. There is also a dusty adjective use, meaning already laid down, but you can live a full English life without needing it. Though actually, when people reach for this word they sometimes want precept (a moral teaching) or script (the written words of a play). Prescript is narrower: a rule set down, with the feel of something inscribed in front of you. If your sentence still works with “rule” or “regulation,” those plainer words are usually kinder to a listener.

規定、訓令。非常書面；日常說「規定」即可。