The most important slot in a report instance is the
definitions one. This is the slot which contains the whole list
of definitions (again, a definition is the abstract description of
something that needs to be documented). Because reports do not make any
assumption on the final organization of the reference manual (that is
the job of the assembly stage of the Declt pipeline; see
Assembly), the list of definitions is completely flat and mostly
unorganized.
In terms of organization, the only guarantee you have is that the library’s main system definition comes first, and that does not even mean first in the list (although it currently does), but first among all system definitions. Apart from that, you shouldn’t count on any kind of ordering within that list.
Also, the list being flat means that every single definition appears as a toplevel element. In other words, it really is a list of definitions, as opposed to a tree of definitions. For instance, things that are normally nested, such as ASDF modules or files (being part of a super-component), slot definitions (being part of a class, structure, or condition), methods (being part of a generic function), etc., are directly accessible in that list. You need to remember this if you ever need to filter out the definitions you’re not interested in.
On the other hand, this list being mostly unorganized and flat doesn’t mean that the programmatic order of things is completely lost. Many, if not all definitions, contain cross-references to other definitions. For example, ASDF system definitions point back to their dependencies, classes to their slots, generic functions to their methods, etc. In all these cases, the original definitions order is preserved.