The idea is that the sovereign can do nothing illegal because by definition it is the fount of legislation. Lawful, therefore, is whatever suits the sovereign.

In the UK this is neatly seen by the Queen being immune, and not needing any documents. After all, why would she need a document asserting that she gives herself the right to drive? She can just say so to the cop that pulls her over. Except the cop cannot pull her over if he knows she's the driver.

Even the constitution is subject to the sovereign, if you think about it, since it is meaningless without the sovereign but the sovereign is not meaningless without the constitution. It's a bootstrapping problem, after all. Why is the constitution valid just because a bunch of blokes signed it and one you sign is meaningless? Because they, the winners of the Revolution, the folks in charge of the militias, the de facto sovereigns at the time, signed it.