From publisher blurb:
This is the 13th Age System Reference document. It contains all the open content from the 13th Age RPG which we call the Archmage Engine. Whether you are a publisher or a gamer, you can use this under the terms of the Open Gaming License.
This is not the 13th Age Core Book, which is a beautiful, gamer-friendly book packed with extra content and written in the author’s distinctive style.
It is however, an excellent, playable game, and we hope it will tempt you into trying the real thing.