If you include folders, tranches and stages, the complete picture of valid relationships between all the project classes in cattaDoc is illustrated - a bit abstract - in this figure:
With folders, the nesting of project classes in cattaDoc can be as deep as you like. It is not limited to 4 or 6 levels.
Notice, that in a digital document and project management system like cattaDoc, a document can be related to more than one project. It is not like in the physical world, where you have to archive the original paper document in relation to one project archive (in a binder or a box), given the fact that there is only one original paper document.