Answer:
Openings are used to create cutouts in surfaces. Finite elements are not generated and surface loads are not applied to the openings. Openings on the edge of a surface are allowed as well.
The boundary lines of a surface must form a closed continuous line. In our example, the warning message only appears for Surface 1, but not for Surface 2. Surface 1 is defined by three boundary lines on one edge, but Surface 2 is not.
A comparison of the results shows that you can model your surfaces with openings in all three cases.
