You can attach a clipping profile to define a mask around a ceiling grid or to make a hole in a ceiling grid. You could, for example, create a mask to define a ceiling grid for an irregularly shaped room, or exclude parts of a room from the ceiling grid.
For a mask, you can use closed polylines and spaces.
Note: To attach a clipping profile, you must first draw a closed polyline or space with the desired boundary or hole shape.