Real Seal of Truth. It's the stickers they put on diplomas and notes of distinction that resemble the "wax seals" that were once melted onto official documents with the purpose of authenticating them.
RSOTs can often be found on fake documents photochopped by W. African internet scammers. A scambaiter laughing at their fake documents would often jokingly praise a scammer's RSOT, especially if it is the only thing on the faked certificate that doesn't contain spelling errors. As in "with an RSOT like that, this's gotta be real." Or "this RSOT just convinced me that he's a real barrister."