https://support.google.com/merchants/answer/160161?hl=en-GB
About unique product identifiers - Google Merchant Center Help
Unique product identifiers (UPIs) define the product that you're selling in the global marketplace. They uniquely distinguish products that you're selling and...
google merchant centerabout uniqueproduct identifiershelp
https://www.developer.ebay.com/api-docs/user-guides/static/trading-user-guide/prod-ids-multi-vars.html
Product identifiers in multiple-variation listings
product identifiersmultiplevariationlistings