A device or user identity cannot be established transparently, such as with non-domain BYOD devices,...

Portal authentication is a user identity discovery method that can be used when a device or user identity cannot be established transparently, such as with non-domain BYOD devices, and allow users to create their own credentials. Portal authentication requires users to enter their credentials on a web page before accessing network resources. The other methods are used for transparent identification of domain devices or users. References: https://docs.fortinet.com/document/fortiauthenticator/6.4/administration-guide/372406/user-identity-discovery