In FreeNAS-11.3-U1, when switching Services UPS UPS Mode to "Slave", the "Driver" field does remain as a mandatory field. In contrast to that, the documentation does (correctly) indicate: "The Driver field changes to Remote Host when UPS Mode is set to Slave. Enter the IP address of the system configured as the UPS Master system."
If I understand NUT correctly, selecting a driver such as usbhid-ups does not make sense on the slave system. This might cause user confusion and connectivity issues.