Some flight boards like the KK2 and CC3D do not have built in FrSky Telemetry settings that are compatible with a D4R-II receiver and Taranis transmitter. The FBVS-01 fills this gap by connecting between the battery and the D4R-II giving the D4R-II the ability to read the battery voltage and send it back to your Taranis. This makes the type of flight controller you are using irrelevant as to whether or not it supports FrSky telemetry.
Referenced Videos:
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Taranis Telemetry Bars:
Taranis, Naze32
Taranis and the KK2 Board:
Acro Naze32, Taranis, & D4R-II Telemetry Setup (CPPM):
Product Links:
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FrSky Battery Voltage Sensor (FBVS-01):
Get your own Taranis:
Taranis XD9 (in video):
Taranis XD9 Plus:
CC3D Flight Board with case: