RJ cables will pass power and data between instruments in your glider typically between your Flarm and your vario.
All the LXNAV S series standalone varios have an RJ12 (6 pin) port on the back. It will be named either 'GPS' or 'Main Power.'
RJ12 to RJ45 (6 pin to 8 pin) cables are often used to connect Flarm units to variometers or flight navigation computers.
LXNAV PowerFlarm and PowerMouse devices have an RJ45 (8 pin) port, so to connect your vario to your LXNAV Flarm you will need to select an RJ12 to RJ45 cable (6 pin to 8 pin).
You will receive an RJ12 to RJ12 (6 pin to 6 pin) cable unless you specify otherwise.
Alternatively we can leave one of the RJ plugs unconnected to make it easier for you to run a cable from front to rear cockpit in 2 seat aircraft.