Law is knowledge of the legal order the character was raised inside — which is rarely one order at all. Clan custom and the vendetta that enforces it, feudal obligation and the lord’s court, royal justice where it reaches, and the dense, jealously guarded regulation of a guild town all operate at once and frequently against one another. Canon law is not covered here; that belongs to the relevant Ritual skill.
The skill is knowledge rather than advocacy. Arguing a case aloud is a Discourse (Law) persuasion test — the law tells the litigant what to say, and Discourse decides whether saying it works. Preparing a writ follows the ordinary rules for composing a document, with Law supplying the subject; such documents are usually dictated to a professional scribe, since being learned in law and being able to write a fair hand are separate accomplishments.
Foreign law carries the same distance penalties as foreign folklore, and a character arguing in a jurisdiction not their own is very often wrong in ways they cannot detect.