# Customer Reference

We track logged in customers across devices by generating the [customer reference](https://docs.nosto.com/techdocs/implementing-nosto/implement-on-your-website/manual-implementation/adding-the-customer-information#tagging-customer-reference) in the [page tagging](https://docs.nosto.com/shopify/guides/how-nosto-modifies-your-theme#amendments-to-include-the-nosto-tagging). The customer reference is generated from a combination of the Shopify customer id and the store’s [permanent domain](https://shopify.dev/api/liquid/objects/shop#shop-permanent_domain).