You create a Bitmap Clipping in ImagePro by drawing a rectangle while holding down both the Shift and Control or Alt key. Then you click inside the clipping and drag it to another application. If you drop the clipping in a Tracker window or on the Desktop, the clipping will be saved as a new image file. If you use the right (or secondary) mouse-button to drag the clipping, when you drop it a menu will be displayed so you can select what format to save the clipping under. If you drop a clipping on an ImagePro window it opens the clipping in a new window.
Please note: The BeOS default image viewer, ShowImage, SEEMS to support the Bitmap Clipping protocol, but it doesn't. You can only drag clippings between different windows of ShowImage itself, and not other applications. At the time of writing I am not aware of any applications besides ImagePro which support the Bitmap Clipping protocol.