Unique product identifiers requirements for Google shopping

Hi, Mark here from Shopify Unique Product Identifiers uniquely distinguish products you are selling and help match search queries with your offers. Product manufacturers assign the UPI's to each product. Common UPI's include Global Trade Item Numbers (GTINs), Manufacturer Part Numbers (MPNs), and brand names. Not all products have UPI's. Depending on the type of product you submit, you'll submit different identifiers. For all your items, it is recommended to submit all three attributes (gtin, brand, and mpn) to help boost ad performance and help users find your products. Some products, such as customized products, don’t have a GTIN assigned or even a brand, so you don’t need to submit one. Be aware that when Google knows that there is a GTIN for your product (a manufacturer/other merchant have supplied GTINs for the same product) they will require that you add it correctly. You may find more information about unique product identifiers and google shopping here - https://support.google.com/merchants/answer/160161?hl=en