Ramon Cortez Let's see if I can explain this the right way. The answer is yes and no, sorta.
Basically, Eloqua uses email address as the unique contact id for all contacts in the database. CDOs are basically separate tables holding contact data where the foreign key mapping them back to contacts is email address. So, every CDO requires an email address.
But the email addresses don't need to be unique for each CDO. You can map multiple CDOs of the same type to a single email address. For example, one user could have multiple sales orders all with different values. Each order would have a different Unique Code Field. In that case, the Unique Code Field would be the order id.
In the case of the integration above, if I was doing it for multiple orders per user rather than one CDO for each user, I would map to "identifierFieldName": "orderID", rather than to emailAddress. But, the CDO still requires an email address. In cases like that, there's actually another mapping you'd use with "EmailAddressField": "emailAddress" (<- I think, you'll have to check the actual EmailAddressField name, that's just from memory). I didn't show that above because the identifierFieldName was actually the email address, so it would have been doubling up.
Does that make sense?