Wings can only work with “closed” geometry
This is only partly true.
Inside Wings the geometry has to be ‘closed’ but an imported model does not have to be ‘closed’.
This mean that you can import a model that is not closed. If you for example have a box with one side removed that you try to import into wings then the removed side will be closed on import and the added suface will be assigned the ‘_hole’ materiel witch by default is a semitransparent blue material.
Now that the model is a ‘closed’ model you can work on it in wings.
Let’s say that you also would like the bottom of the box to get removed.
Well I suppose you allready guessed it, just assigne the ‘_hole’ material to the bottom face. Now when you done with you model and export it you will get a box open at both the top and bottom because the faces assigned the ‘_hole’ material won’t export.
So Wings can handle none ‘closed’ models by temporary ‘closing’ them inside Wings.
What it can’t handle is , and here I Quote the Wings developer Bjorn:
Wings uses the winged-edge data structure (WE) internally. The WE can only represent a well-ordered polygon mesh where each edge has one face on each side of it
This should probably be read as an edge only can have two faces connected to it. Nothing less and nothing more.
On import Wings tries to fix this by temporary welding and adding of faces and by using the ‘_hole’ material and it works well on most models.