The PA-40 W-2 RW is a schedule included with your Pennsylvania tax return that reconciles income on your W-2 to make sure the correct amount of wages have been reported. It's required when the amount listed in box 5 of your W-2 (Medicare wages) is higher than box 16 (state wages).
This can happen when you have income items that are taxable for Medicare but not state tax purposes, such as contributions to certain deferred compensation plans.