From left to right in the picture below:
1. Cab Bus Cable with RJ12 connector 6P6C. A flat 6 wire cable used for tethered cabs, UTP panels, the cab bus and the Power Cab. For regular cabs the maximum length is 40 feet. When used with the Power Cab this cable carries full track power so the maximum length is 8 feet.
2. Cab Bus Cable with RJ12 connector 6P4C. A 4 wire cable 6 feet long used for tethered cabs. Notice the 6 pin connector with only 4 slots being used. This is a regular Cab cable. No track power.
3. Control Bus Cable with RJ-H connector 4P4C. A 4 wire cable used between boosters and a command station. Maximum length is 300 feet.
The two outer wires (white and blue) on the six wire cable are used to provide a track power in the output Power Cab.
The rest of the time the two outer wires are not used at all. That includes using it for the cab bus, RB02, and the direct connection from the RB02 to the RPT1.
For the records I am NOT a fan of the cheap telco splitters. They can and DO fail. I have taken MANY calls where the source of problems was traced back to these things.
They must be of the 4 wire variety and NOT have flipped or swapped colors. Use these at your own risk.
The end result is all that matters.
You need clean data on pins 3 and 4 (red /green) .
You need a minimum of 8vdc / nominal 12vdc on pins 2 and 5 (black and yellow).