In Windows Active Directory, a User Principal Name (UPN) is the name of a system user in an email address format. A UPN (for example: john.doe@domain.com) consists of the user name (logon name), separator (the @ symbol), and domain name (UPN suffix).

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A User Principal Name (UPN) is an attribute that is an internet communication standard for user accounts. A UPN consists of a UPN prefix (the user account name) and a UPN suffix (a DNS domain name). The prefix joins the suffix using the "@" symbol.

A UPN consists of a UPN prefix (the user account name) and a UPN suffix (a DNS domain name). The prefix is joined with the suffix using the "@" symbol.

When you synchronize on-premises Active Directory users with Azure, Office 365, or InTune, the User Principal Name (UPN) is often used to identify the users. This means that all users that will be synchronized should have the userPrincipalName attribute assigned, and the values should be unique in the Forest.