From the website:
This is the core rulebook of Alternate Realities, and describes the Primary Reality... our own.
From the Introduction:
As the name implies, Alternate Realities is a role-playing system which allows players to explore virtually any conceivable setting. However, in order to establish a "baseline" or frame of reference from which to describe the many worlds of AR, we've found it helpful to designate one such world - the "real" world - as the Primary Reality. This doesn't mean that most (or, really, any) AR play is expected to take place in this reality (after all, we can do that at home!), but rather it means that all other realities are defined by how they differ from the real world.
This book presents the systems and baselines from which to develop your own game using the Alternate Realities systems.