The FortiGate unit supports both SSL and IPSec VPN technologies. Each combines encryption and VPN gateway functions to create private communication channels over the Internet, which helps to defray physical network costs. Both enable you to define and deploy network access and firewall policies using a single management tool. In addition, both support a simple client/user authentication process (including optional X.509 security certificates). You have the freedom to use both technologies; however, one may be better suited to the requirements of your situation.
In general, IPSec VPNs are a good choice for site-to-site connections where appliance-based firewalls are used to provide network protection, and company sanctioned client computers are issued to users. SSL VPNs are a good choice for roaming users who depend on a wide variety of thin-client computers to access enterprise applications and/or company resources from a remote location.