User who initiated the request from the client system.
If the user is found either in the system's user management or in a connected LDAP directory, the request runs under this user's roles.
If the user is not found, the request is only granted the basic 'I_EVERYONE' role. The user name may be used for logging purposes in this case.
Actual authentication is handled separately via the request's headers, so no password is required in the request's data.