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If you go up to the Help Center and search for PBS Glossary you will find a list of commonly used abbreviations.
RWAP or RWP = Received with a Problem: when you get a book that was unpostable, posted wrong, damaged, or didn't meet your a member's Requester Conditions (RCs)
ARC = Advanced Reader Copy: also other forms of Uncorrected Proofs. They are not swappable on PBS because they may not be the final version of the book released. They can be offered in the Book Bazaar and swapped unofficially though.