The barcodes printed at the bottom of receipts are not defined by any standard. They exist only for internal use by the stores.
The barcoded data usually exists to identify a particular transaction, enabling a future lookup of the transaction to perform a return, or to perform a shipping function, or to serve as a kind of message digest validating that a receipt was generated by the retailer. Sometimes the barcodes serve as an entry code for a customer survey or for a prize drawing. Some barcodes may simply contain the URL of the retailer.
Each POS system generates their own unique barcodes. The code could contain a randomly generated UUID, or it could have a combination of date and transaction number. The symbology it is printed in could be Code 128, Code 39, PDF-417, a QR Code, or even a proprietary symbology.
The barcodes are not fixed - if a retailer upgrades their systems, they might produce different codes tomorrow.
Finally, if you are thinking of creating a "receipt storing app" for an iPhone, know that most retailers require the original paper receipt for returns, and will not accept a copy. Part of the cashier's task may be to validate the paper is genuine receipt tape pre-printed with the store's logo. It may be their process requires them to use a marker to strike-out the items that were returned. Receipts are controlled by the retailers because copies of receipts are used by shoplifters to return stolen merchandise.