In terms of the debugger architecture, a port:
Is a container for a set of processes running on a server. For example, a port might represent a connection to a Windows CE-based device by a serial cable, or to a networked non-DCOM machine. One special port, called the local port, contains all the processes running on the local machine.
Can identify itself by name or identifier.
Can enumerate all processes running on the port and launch and terminate these processes.
Visual Studio supplies a default port that handles all Windows-based processes, native and managed. A custom port must be implemented for connections with external devices that are not Windows-based. To supply such custom ports, a custom port supplier also needs to be implemented.