Fluid Topics supports required authentication: this means it will only be accessible to registered users. In this case, all guest users are invited to log in. If there is only one realm which is a SSO one, then guest users will be automatically redirected to the SSO. Otherwise the authentication form will be displayed.
This option can be configured in Fluid Topics, in the Administration > Users > Authentication interface.
Some special use cases require SSO authentication for users and login/password authentication when calling the Fluid Topics web services. In such cases, the login/password realm prevents the portal from redirecting automatically to the SSO.
The Credential form option can be used to disable login/password authentication in the interface (it will remain accessible through web services).