Chapter 3 Authentication for FortiOS 5.0 : Authentication servers : RADIUS servers : Microsoft RADIUS servers : RADIUS user database
  
RADIUS user database
The RADIUS user database is commonly an SQL or LDAP database, but can also be any combination of:
usernames and passwords defined in a configuration file
user account names and passwords configured on the computer where the RADIUS server is installed.
If users are members of multiple RADIUS groups, then the user group authentication timeout value does not apply. See “Membership in multiple groups”.