We usually supply barcode numbers in the 13 digit EAN-13 format, which has 0 at the start (eg 0799439112766 ). But, 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 (see the picture below).
Outside of North America it is common to print the barcode on products in the EAN-13 format. However, 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 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.