In architecture, what does it mean to fill the wellhead position?

GB/T5000 1200 1 Drawing Standard stipulates the legend of structures and accessories, in which the drawing of holes in architectural drawing is represented by filled shadows.

This screenshot of the above architectural drawing shows a hole, and the shadow filled in this part of the drawing is made according to the unified standard of architectural drawing, which clearly shows that this is a standard representation of a through hole.