The partial screen record button (the dashed rectangle icon) is located to the left of the main record button. When clicked, the screen will darken and a rectangular selection will appear. This capture area can be resized using the mouse. While the rectangular selection is present, the user cannot interact with any windows on their desktop, and the mouse is now controlling the sizing of the region.There is a control strip at the bottom of the focused screen where you can cancel out of this mode, start the recording, or use presets to constrain the region you are capturing. (See Partial Screen Capture for more detail).