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 organisation 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 username can be reclaimed from the user so you can use it.
Managed Apple IDs: if a different organisation 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 organisation has a valid claim to the domain, no organisation can federate it.