Before the answer of Michael I can say this little thing :)

Nurbs are curves who define surfaces who define a volume :)
You must use these 3 steps (except if primitives are yet exist)
It's like an "envelope" a skin
There is no "density" :)
This density can be calculated yes, but not at first.

Solid modeler define a volume who has directly a "density" like a "stone" and parametric values
In theory you can't erase a face of a parametric volume for see that is exist in the interior else that is a "concrete, real, solid" volume :) (I am not sure of that as never used a solid modeler :)
In nurbs modeler yes, volume inside is always "empty" except if you have put inside an another "empty volume" etc (like russian Doll :)

There is a different style of CAD software called "parametric solid modeler"
Some examples of this are SolidWorks, Alibre, Inventor, Pro/E, etc...

MoI is not really set up to do exactly the same thing as it has not this sort of parametric function of creation :)

and an enlighting Photo :)
http://moi3d.com/forum/index.php?webtag=MOI&msg=207.5

refound a little discussion with Michael :)
> Some precisions : What is the status of the "primitive solid"? (Cube, sphere, cylinder...
> Are they a "Volume" or a "surfacic" object?

They are a volume that is defined by the Boundary-Representation (B-Rep) method, where there is a connected, closed skin of surfaces that partition space into an inside region and an outside region. Having a concept of "inside" versus "outside" is what makes something a volume.

> And the Status of Moi ? Surfacic, Volumic, Solid (I suppose not:)

MoI is what is sometimes called a "hybrid" modeler, where you can have both surfaces and volumes/solids (do you consider a volume and a solid to be different things? I would categorize them as the same). All objects are made up of surfaces, and if the surfaces are joined together at their edges and form a fully closed skin, then that defines a solid object that has a volume.

MoI allows you to separate a volume into its "skin" and work on the skin as individual surfaces. Then later on you can join the skin back together to recreate the volume. So in this way you can work both with surfaces and with solids/volumes.