Hey @michael_soprano
One schema can have multiple identities defined in it. When you enable that schema for profile, you will have to define one of these as primary identity (if not using identity map to pass the identities). This primary identity will have to be populated in every row that you send through, but you can also have more than one identity sent through in one data row. For instance, usually for web streaming data, ECID is defined as primary id because you might have rows where you would just have that identity available, for instance, unauthenticated users. But, when these users login, you might get more identities along with ECID to pass along, like a backend CRM Id, email etc.
All the above identities then form part of the identity graph of a profile.
Hope the above helps.
Cheers,
Abhinav