This release fixes bugs in the previous 3.6.x firmwares. See the change list at https://github.com/Duet3D/RepRapFirmware/wiki/Changelog-RRF-3.x#reprapfirmware-364.
The Duet 3 IAP files (the file that needs to be on the SD card in order to install a main board firmware update) are updated to DuetIAP 3.7.3, which is the version the 3.6 package feed installs. The Duet 2 IAP file is unchanged.
For WiFi equipped boards the recommended WiFi firmware version is 2.4.2 available at https://github.com/Duet3D/WiFiSocketServerRTOS/releases/tag/v2.4.2.