In Microsoft’s Active Directory the User Principal Name (UPN) is the unique sign in name or username, that uniquely identifies a user in the Directory.
Microsoft uses Azure Active Directory (Azure AD) for all it’s online business services (like Microsoft 365, Office 365, Dynamics 365, Power Apps, Azure, etc.)
All users will have a sign in name, the name they use when they sign in (log in) to their accounts. This name is their User Principal Name.
The User Principal Name is formatted like an email address (firstname.lastname@example.org) but, not always the same as the user’s actual email address.
By default, when an administrator create a new user in Microsoft 365 (online), they specify the User Principal Name, which will also set the default email address but, this can be changed later.
When prompted for a User Principal Name or Sign in Name, it is important to be use whether a user’s UPN is the same as their email address or different.
To be sure, always ask your user to specify their UPN by, for example, asking "What is your full username you use when signing in to Microsoft’s online services like Office 365"