You need to buy a unique GS1 Company Prefix and number series from GS1, to be able to create GTINs for your products. This GTIN must then be included in all documentation that is linked to the various products. For example, in catalogs, price lists, invoices, etc. The information requirements from the buyer are stated in the agreements – project or framework agreements – that you sign.
When it comes to messaging standards used in digital communications, such as BEAst’s various messaging for business transactions, all have GTINs as identifiers. There are also fields for the supplier’s trade item number and the manufacturer’s trade item number etc. but the requirement is that the GTIN must always be included.