UPC-A Barcodes are effectively a subset of EAN-13 Barcodes. If the first digit on the EAN-13 number is a ‘0’, then the bars will be of both the EAN-13 and the UPC-A (without the leading ‘0’) will be identical. The displacement of the human readable numbers below differ between the UPC-A and EAN-13 barcodes however, this is the biggest difference. Both barcodes can be scanned by the majority of scanners easily.
UPC-A Format barcodes have traditionally been used in the USA, whereas EAN-13 format barcodes have been used throughout the rest of the world. Nowadays, the majority of stores throughout the world accept barcodes in either format. Howevers there may be some older systems that only accept one or the other. This means that if your product is being sold in the USA , the UPC-A format barcodes are best, however if your product is international, or sold in a country other than the USA, an EAN-13 Barcode is best.
If you come across a store that has difficulty reading your EAN-13 or UPC-A Barcode, they can either ignore the leading ‘0’ or add a leading ‘0’ depending on how many digits their system prefers. If this is done, the barcode will read exactly the same as the opposite format (as the bars are identical regardless), and will still be globally unique.