A WiFi mesh can provide access to widely-distributed clients. The root mesh AP which is directly connected to the WiFi controller can be either a FortiAP unit or the built-in AP of a FortiWiFi unit that is also the WiFi controller.
An alternate use of the wireless mesh functionality is as a point-to-point relay. Both wired and WiFi users on the leaf AP side are connected to the LAN segment on the root mesh side.