What is the second RJ-45 connection used for?

The VX-VEB160G4 Ethernet Extenders can support 2 separate LANs through CAT3. These ports are also isolated from each other. This feature provides a cost efficient method to extend 2 separate LANs while using one connection.
For example, users might want to extend the Management LAN and a Guest LAN to a remote location over one copper pair and prevent the 2 (both) from communicating with each other.