The Rectangle / Square (Annotation) tool allows drawing a rectangular annotation shape on the selected medical image. The drawing is performed by dragging with the associated mouse button over the image, and releasing the mouse button once you traced the desired rectangular shape. Keeping the SHIFT key of the keyboard pressed will force the shape to be exactly square.
The Tool Options panel associated with this tool is shown in the picture below:
In addition to the options already described for the Line (Annotation) tool, it is possible to enable the Filled shape option to fill the internal area of the rectangle with a solid color.