PLASTIC stands for the PLatform for AStronomical Tool InterConnection. It is a protocol which allows tools on the desktop to communicate with each other. Very briefly, the way it works is that applications can send messages to a central hub process which will then pass them on to other applications which can respond to them in some appropriate way (GAIA supports image display and position tracking). You will require a hub, of somekind, to use this.
You can find more information about how it works, what applications are compatible, and how it can be useful at the PLASTIC web site. You should note that PLASTIC is an evolving system, so future compatibility isn't guaranteed. To remove the Interop menu you can use the Startup options... window located in the File menu.
GAIA -- Graphical Astronomy and Image Analysis Tool