The RJ48C pinout utilizes pins 1,2,4 & 5 on the connector. A RJ48X connector is wired like an RJ48C but includes shorting bars across the pin pairs.  When nothing is plugged into the jack the shorting bars loop back the line towards the far end (pin 1 shorts to pin 4 and pin 2 shorts to pin 5). This is ideal when the far end equipment has loop back testing capabilities.