The FocusBug has two shapes, round or planar. The bug shape is controlled in the aperture pane from the shape pop-up. The FocusBug tool changes appearance from a round body to a square body depending on which shape is used. The round shape is the default. This creates a round or oblong sweet-spot. This is similar to using a selective focus lens. The bokeh will extend to all sides of the image.
The second shape, planar, simulates a tilt-shift or view camera appearance. It creates a sweet-spot that slices through the image from one side to another.
Round FocusBug
Round FocusBug with grid visible
Round FocusBug with grid and mask visible
Planar FocusBug
Planar FocusBug with grid visible
Planar FocusBug with grid and mask visible
You can mix and match FocusBug shapes to create just the look you desire. See the Using Multiple FocusBug section for details.