FortiAP devices are thin wireless access points (AP) supporting the latest Wi-Fi technologies (multi-user MIMO 802.11ac Wave 1 and Wave 2, 4x4), as well as 802.11n, 802.11AX , and the demand for plug and play deployment.
This article explains a useful command to determine what IEEE 802.11 standard is being used between a connected workstation or mobile device and a FortiAP.

The command output shows the standard being used, this is useful when troubleshooting download and channel overlapping issues.

Forti_1 # diagnose wireless-controller wlac -d sta

* vf=0 wtp=2 rId=2 wlan=Rock_zone vlan_id=0 ip= mac=08:5b:0e:c6:46:1a vci= host= user= group= signal=0 noise=0 idle=0 bw=0 use=4 chan=108 radio_type=11AC security=wpa2_only_personal encrypt=aes cp_authed=no online=yes

The 'radio_type=11AC' confirms that the device is using the 802.11AC IEEE standard.