The TCPMapping filter API lets you intercept TCP connections and data. You can modify the data, make additional connections and send data to the server or client side. Whilst other filters are high-level protocol specific (HTTP, POP3), TCP mapping filters monitor at the TCP level allowing more fine grained control.
In order to monitor TCP connections there must be a TCP Mapping service running in WinGate as this service is the one that accepts registration of the filters and pushes events to them.