CompareSubmissions returns two of the contest's submissions and where they say the same thing: both sources, and the runs their normalised token streams share as line ranges on each side, longest first.

It is what a copying case is read with. The case names two submissions and how much of the shorter one they share; this says which part, without a reviewer hunting for it in two files.

Both submissions must belong to the contest, and the permission asked for is the contest's: whoever may review its submissions may compare them. Nobody needs rights over the submissions anywhere else, which is the point of asking judge rather than atlas.