What
"A RUP Plug-in is a deployable unit for one or several RUP process components that can be readily "dropped" onto a RUP Base to extend it. A RUP Plug-in can be compiled into a physical file, allowing it to be moved around and added to a RUP library with a compatible RUP base."
The Rational Unified Process Made Easy Per Kroll, Phillipe Krutchen. Addison Wesley