||It is a kind of like proxy, but not quite. Let's say you want to use regular PC^2, (pc2admin, pc2judge [autojudge], pc2team, pc2scoreboard) to setup a local contest for practice (couple of teams) but without input/output data you are not able to do this. What to do? Input/Output data is not available for problems.
You can use external validator in PC^2 which will submit team's code to online judge and fetch verdict as-if you actually judged that solution on your computer. In the end you get a ranklist just like a normal PC^2 contest.