- UPN is the User Principal Name. It is always in the format which looks like an email address. Essentially it has 3 parts. User account name, the separator (i.e. @ symbol) and UPN suffix or Domain name. Its primary purpose is to use during the authentication. Whereas the Name is supposed to be for display purposes. The Name and UPN can be same or different. Depending upon your objective you should use one vs the other.
Refer User Name Formats
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The first answers the second question to some part. UPN will change based on the domain. Domain is the UPN suffix. The Name is the display name and may not change unless you specify the rules when migrating AD users from one domain to another.
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NameIdentifier is the unique "SAML name identifier of the user". In other terms its just the ID of the user object. Purpose: When trying to identify a user uniquely this should be your go-to choice.
Refer:
