The TBLM1 is a companion device for LISNs, which splits the conducted noise signal into its common mode and differential mode components. The LISN Mate is connected to the output of a LISN inserted into the positive supply line and to a second LISN inserted into the negative supply line. The conducted noise signal at the output of a LISN is the sum of common mode and differential mode noise.
The LISN mate splits it into the common mode and differential mode components, with each signal component available at a separate BNC port.
You will find the specifications of the TBLM1 LISN here.