Important note; the version of the NPC realms presented in the P&H represents something of an 'ideal' version of the realm. Things are in an equilibrium, there are no major changes or conflict ongoing. Holdings are filled. Etc.
If, any only if, PC domains extend into the are, then that realm will be updated, taking into account recent events, and making some available holding slots (not very many, but a few - in RoE vacant slots are the exception, not the norm).
