A hollow tube as a referenced in my PM reply, which I thought probably would need further clarification, is one that is hollow and the cavities are formed by using fabric dividers. The cavities can be separated by just fabric or thin nylon or foam rubber in varying thicknesses.
The other type of cases are multi-tube which are made up of several tubes bundled together in whatever shape the case maker desires, and foam core which is made of a block of hardened compressed foam with the cavities molded around steel rods.