This is not a duplicate question about file-system hierarchy that can be answered by pointing me toward
man hier, ubuntu-policy, or something. I've looked at those questions. My problem is that I need a deeper look at the directory structure.
It turns out that the answers to many questions are in the purpose of lower level directories like /usr/share/applications or /var/run/sendsigs.omit.d, which do not seem to be covered by the standard documentation that is referred to in the questions I've found. I'm sure documentation exists concerning all these directories, but since they are standard to Ubuntu, it would be nice if a hierarchy document covered all their various purposes, perhaps even with links back to the documents to their related services/applications.
The reason why I want this is because it cuts down on time and confusion spent looking through Google etc. when I encounter a problem if I already know where directories associated with some problem reside.