The YP doc set and product have evolved to the point where the YP Reference Manual needs to be a manual that contains "lookup" information only (e.g. class descriptions, task descriptions, variables, etc.). Information such as migration, build concepts, and other non-"lookup" information needs to be removed and put into the new YP Concepts manual (see Bug 12370).
The concepts from the ref-manual have now been moved and reside in a new manual (concept-manual). See http://www.yoctoproject.org/docs/2.5/concepts-manual/concepts-manual.html. Scott