Have you ever tried connecting an RJ11 cable to your phone or wall Jack and you realize you have the wrong connectors on the cable and they just wont fit?
On occasion we do come across this question of why their connector has an “offset” tab rather than a center tab on the plugs. This is because you have a modified modular connector, aka MMP or MMJ (Plug or Jack) and its clearly recognized by its locking tab, which is offset from the center of the plug body compared to being centered on the standard type of modular plugs. These connections were often found on equipment manufactured by the Digital Equipment Corporation (DEC), which is no longer.
Below you can see how the tab is offset on the modified modular plug.
The purpose being so standard modular connectors found on ethernet and phone cables cannot accidentally be plugged in.