Tours are used to aggregate other document instances, especially topics. They are of a means to take an alternative walk through the documentation.

A generic document is like a topic, but has no default storage space. It is usually a subdocument of another document. Use this doctype if you do not want the document instance show up in an index.

Modules help to create modular content for single sourcing. These modules are transcluded by topics (and instances of other document types).

Sections do not stand for their own. That is, if the parent document is removed, section documents are also removed (or their information is moved to other documents where the fit better according to recent understanding).