Resolve Apple ID conflicts in Apple School Manager
The domain, or domain name (as it is also commonly known), is the name that designates the larger organization rather than an individual member. Domain names are registered and must be globally unique.
When you configure federated authentication, Apple School Manager checks to learn whether your domain name is already part of any existing Apple IDs:
Apple IDs: If someone else is using an Apple ID that contains the domain you want to use, that Apple ID user name can be reclaimed from the user so you can use it.
Managed Apple IDs: If a different organization has Managed Apple IDs in the domain that you want to use, Apple will investigate who owns the domain and notify you when the investigation is complete. If more than one organization has a valid claim to the domain, no organization can federate it.