According to the Adobe documentation, primary identity is an identity that must be set in a schema in order to use it in Real-Time Customer Profile. Primary identity is an identity that uniquely identifies each profile within a dataset and across datasets within a tenant. Primary identity can be either predefined or custom, but it must have cardinality of one-to-one or many-to-one. Reference: https://experienceleague.adobe.com/docs/experience-platform/profile/identity-service/schema.html?lang=en#primary-identity