This release candidate fixes bugs in the previous 3.6.x firmwares. See the change list at https://github.com/Duet3D/RepRapFirmware/wiki/Changelog-RRF-3.x-RC#reprapfirmware-364-rc1.
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 3.7.2, which is the version the 3.6 package feed installs. The Duet 2 IAP file is unchanged.
The WiFi module firmware has not changed, current version is still 2.3.0. WiFi module firmware version 2.4.1 is also compatible with this release.