The TCPMapping filter API lets you intercept TCP connections and data travelling through a TCP mapping server. 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.