Each Content Pack other than the Core Rules depends on one or more Extensions and is intended to adapt the Core Rules to fit a particular game setting (fantasy, sci fi, etc.).
Content Packs exist to help GMs translate People, Places, and Things from other systems into No-Dice Actors.
They:
Content Packs may depend on Extensions.
Extensions never depend on Content Packs.
This one-way dependency keeps the ecosystem sane forever.
Core Rules
Are the always-on content that is generic, genre agnostic, and can be used in any game setting, with or without another Content Pack.