If you look at UltraVNC (http://www.ultravnc.com), they incorporate an option to connect to a VNC "Repeater" via an "ID: " field on the server and viewer. This is incredibly useful for situations where both server and viewer are behind a NAT/firewall, because the server is creating an OUTGOING connection to a publicly accessible "repeater" (connection relay), bypassing the need to open ports, etc. The viewer does the same.
The ID: field is a simple number to track multiple connections on a repeater, to bind viewer and server.
The Windows UVNC viewer has ID: support, but Linux VNC/TSClient does not. Many people use UVNC repeaters, and tsclient incorporating support for repeater connections would make a LOT of people happy, as we would be able to ditch the most common answer of "just use uvnc viewer with Wine." which is a very flaky solution.