RJ ... 'Registered Jack', telephone connection interfaces, registered with the FCC, the U.S. Federal Communications Commission. Their history goes back to the Bell System and AT&T's Universal Service Order Codes (USOC) in the 70s. These were adopted later as part of the FCC regulations. The RJ48 has an extra tab so that a RJ48 plug will not push into a RJ45 jack, but a RJ45 plug will mate with a RJ48 jack.