On Windows and Linux, faculty members have the option of sharing a region. When choosing region, you will see a dashed outline of the desktop sharing area when BigBlueButton Screenshare runs.
Clicking ‘Start Sharing’ will start sharing the selected region. As with sharing full screen, a thumbnail version of what the remote students see will appear in the Desktop Sharing Presenter’s Preview window.
Due to the limitation of underlying components in the new screen sharing, there is no option on Mac OS X to share a region – it will only capture full-screen of the first monitor.