Answer:
This note always appears when a line is loaded in a surface, but is not integrated into the surface. In most cases, the combination is displayed not only with Error 10305, but also with Error 10306.
It is then necessary to integrate the line manually into the surface. You can do this in the "Edit Surface" dialog box, the "Integrated" tab (see Image 03).
In general, this measure is only necessary for "non-planar" surfaces because the lines, nodes, and openings of planar surfaces are automatically integrated into the respective surfaces.