Smart jack

A "Smart Jack" is usually the point at which the telephone company hands-off the circuit to the customer.  This is also referred to as a "demark" (or demarc or point-of-demarcation).  It also had the roll of acting as a control and trouble-shooting device for the phone company.  For instance, the tech on the phone company side can communicate with the smart jack.  She can put the smart jack into a Loopback mode which would allow her to run diagnostics on the span.  This helps ddetermineif any trouble in the network is on the phone company side or on the customer side.