Hi @Maxim-CADman77. I could be wrong, but I believe that is when you have a single solid body, which has an internal empty space within it. In a case like that, I believe the body will have multiple FaceShells, and one of them will be void.
Edit: I just attached a 2024 part with an internal iLogic rule as an example. It contains a rectangular extruded solid, with a rectangular extruded cut that is entirely within the only solid body. The rule gets that body, and loops through its FaceShells collection Logging the result of FaceShell.IsVoid to the iLogic Log window. It returns 1 False and one True.
Wesley Crihfield

(Not an Autodesk Employee)