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Just what it says. Have the Avignon (anti-)popes Clement VII and his successors recognized by as the legitimate papal line. Urban VI and his successors are to be viewed as antipopes. You can have Benedict XIII follow Clement or choose someone else. The papacy may return to Rome or stay in Avignon -- your call.
No cheating -- you can't have some much latter council retroactively denounce Urban and the rest while recognizing Clement and friends. The legitimacy must be achieved by 1450 or so.