Am I right in thinking that an object is solid if it is completely watertight, and all the faces are ‘white side out’?
Keeping it very simple, here are two examples (in the attached jpg), one is a cube (actually drawn as 2’ cube) with no wall thickness specified.
The other is a 2’ cube with the centre pushed out, to leave 2" thick walls all round. One view shows the whole objects, the other a section through them.
If printed (suitably scaled down!) would the cube be solid, and would the open ended cube have solid walls?