These pages are internal developer documentation for Confluence. The main audience for these documents is Atlassian developers, but hopefully plugin and extension developers might benefit from knowing more about how the application works. There are, however, a few caveats:
These documents should give you some understanding of how the Confluence code-base is structured, where to find things, and where to put new things.
These documents are more general descriptions of How We Do Things Around Here. It's a good idea to be familiar with these documents, but keep in mind that no rule is set in stone, and the existence of a guideline does not absolve you from your responsibility to think.