We normally supply barcode numbers in 13 digit EAN-13 format, with a leading 0 (eg 0799439112766 ). However, these numbers can also be used in 12 digit UPC-A format without the leading 0 (eg 799439112766 ). The actual bars produced for both of these are identical.
Outside of North America it is common to print the barcode on products in the EAN-13 format. But keep in mind that some retailers have software systems that don’t like numbers with a leading 0, and hence they will prefer to use your barcode in the 12 digit format (eg 799439112766 ). This is still fine – the number is still the same and still unique and will work OK. You can let the retailer work out which version of your barcode number they will use.