As Yurisk already said, there are some ways how to authenticate even computers.
Possibilities are quite wide. Beside mentioned ones you can for example has your users logged to FAC and allow them to enroll their own device certificates .. you can limit how many devices they can enroll .. and then they can do 802.1x EAP-TLS wired or wifi auth .. for example.
Beside mentioned fact that FAC is in most basic form huge RADIUS server .. it's true, but more precisely it is auth concentrator and centralized point.
As for mentioned SSO it can read user data from many sources (RADIUS Accounting, Syslog, various Windows AD methods, FortClient with SSOMA (mobility agent modul in FortiClient) .. , process those logon data and pass them via filters and those fitting to specific need push to connected FortiGate units.
But besides RADIUS and FSSO, it is also token handling platform (FortiTokens, Ubikeys directly, 3rd part like RSA via auth chaining to the RSA server, even combining those 3rd party with LDAP/AD to single auth), SAML, OAuth, RADIUS Proxy (not only for Accounting data, but also RADIUS Authentication as it can use another RADIUS server as backend, not just LDAP). Speaking of LDAP and in general, but it's usually used with AD, FAC can sync users based on some LDAP Filters and sort them to groups on FAC, alternatively assign tokens (FortiTokens/SMS/Email) to those users automatically during sync. Also keep user list so once you remove user from LDAP and/or once user stop matching the LDAP filter, that user can be automatically removed form FAC, and if he was provided with token that one will be recovered back to pool of free/available tokens (useful especially with Mobile tokens where you do not need to collect hardware token, which obviously is not possible automagically by FAC itself).
So, possibilities are pretty wide. It more depends on what do you truly need, and there even for specific task is usually more than one way how to achieve that.
Therefore you should be a bit more specific.