The RB-LI2 stereo line isolation unit is used to isolate audio signals from inter area ground hum loops, which could be caused by equipment being powered by different mains power supplies, or different phases on the same supply.
The input and output are connected together through a transformer, which has internal jumpers allowing the outputs to be balanced about ground.
There is also a loop-through output, so that the RB-LI2 can be inserted into a line, forming a transformer balanced distribution point.