GTINs come in 8, 12, 13 and 14 digit formats, which is why the same family includes the UPC used in North America and the EAN used elsewhere. A brand owner licenses a company prefix from GS1 and assigns a GTIN to each product variant, so a different size, flavor or pack count gets its own number.
Retailers and marketplaces use the GTIN to match a supplier product to their catalog, so a missing, reused or mistyped GTIN is one of the most common reasons product data is rejected or listed against the wrong item. Checking GTINs, including the check digit, is a cheap first step before supplier data goes live.