To enable guest portal services on FortiAuthenticator, you need to configure a portal policy that defines the conditions for presenting the guest portal to users and the authentication methods to use. You also need to configure at least one post-login service that defines what actions to take after a user logs in successfully, such as sending an email confirmation, assigning a VLAN, or creating a user account. Configuring a RADIUS client or an external authentication portal are optional steps that depend on your network setup and requirements. References: https://docs.fortinet.com/document/fortiauthenticator/6.4/administration-guide/372404/guest-management

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

The inbound proxy setting provides the ability for FortiAuthenticator to determine the origin source IP address after traffic passes through a proxy for system access. The inbound proxy setting allows FortiAuthenticator to use the X-Forwarded-For header in the HTTP request to identify the original client IP address. This can help FortiAuthenticator apply the correct authentication policy or portal policy based on the source IP address.

References: https://docs.fortinet.com/document/fortiauthenticator/6.4.0/administration-guide/906179/system-settings#inbound-proxy

Realms are a way to delegate user accounts, user groups and permissions per domain when they are authenticating on a single FortiAuthenticator device. A realm is a logical grouping of users and groups based on a common attribute, such as a domain name or an IP address range. Realms allow administrators to apply different authentication policies and settings to different groups of users based on their realm membership.

References: https://docs.fortinet.com/document/fortiauthenticator/6.4.0/administration-guide/906179/user-management#realms