Clean vertical hole: When drilling a round hole in the side of a molding object, consider that the top of the round hole will fall in due to the lack of support. Ensure that the vertical height of the round hole can be output accurately. (Reference: *1)

Circle mid-air hole: Used to drill a counterbored hole in the bottom of the part, allowing the 3D printer to easily bridge the part without support. (Reference: *1)

Hexagon mid air hole: Used to place hexagonal nuts on the bottom surface of the object, allowing the 3D printer to easily bridge the object without support. (Reference: *1)

Reference 1:https://github.com/gregsaun/maker_cheatsheet/blob/master/3d_printing/techniques.md