Guide required to explain content model

There are a number of different kinds of Confluence content. Examples include spaces, pages, blogs, page comments, inline comments and custom content (contributed by Connect apps).

App developers would benefit from a guide explaining the relationship between these different kinds of content.

Such a guide, for example, would explain if it makes sense that custom content can be created in a different space than its container as explained in https://community.developer.atlassian.com/t/custom-content-can-be-created-in-different-space-than-its-container/47829.