UPC-A is the twelve-digit linear barcode used on retail products in the United States and Canada. It predates EAN-13 and is formally a subset of it: prefix a UPC-A number with a zero and you have the equivalent EAN-13, which is why scanners and product databases treat the two together.
For loyalty it shows up the same way EAN-13 does, as an inherited member number from a plastic card program or a point of sale that was configured around retail symbologies. Keeping that number means the till lookup, the receipt and the reporting all continue to work unchanged when the card becomes a wallet pass.
It is numeric-only and fixed length, with a check digit in the final position. There is no room for letters, prefixes or a value that rotates per visit, so it suits a static identifier and nothing more.
Google Wallet renders UPC-A on passes. Apple Wallet supports QR, PDF417, Aztec and Code 128, so an Apple pass carries the same twelve digits as Code 128 or a QR code. QR and barcode identification covers running both wallets against one scanner estate.