The tiers should just match up with whatever discounts existed previously on the cards. Mine says "A" and was a signature plus pass, so I expect "A" to correspond to 20% off merchandise at Disney-owned locations and 15% off food at Disney restaurants, with all of the usual exceptions of course. A signature pass gets the same discounts so should also have been assigned "A". A "B" discount would correspond to all the other passes (deluxe, So Cal, So Cal select, and Flex) I would think since they came with 10% off food and merchandise.
ETA: Corrected discount amount.