From the introduction:
The Northern Realm is a large piece of work, but is based upon a few conceptual and narrative ideas. A modified version of the d20 rules works to accomplish those goals within the game. While the main work has been designed and organized to facilitate easy reference, this document hopes to provide a quick rundown of the game’s setting, rules, and play philosophy in order to aid players and story masters in participating in and designing adventures in the Northern Realm.
This concise introduction to the basic concepts of the setting is presented in broad outline. Usually, it is distributed as a separate document with the core book.