As you might expect, a lot of the information in the document is necessarily technical, but it's interesting for all of us because explains how the individuals modules will interact with the main frame of the phone—the part Google refers to as the Endo. For instance, it sounds like it's going to incredibly flexible:
Users of an Ara phone will be able to power their device with one or multiple batteries; they will be able to swap a depleted battery with a fresh one, without powering off their phone; they will be able to charge one or more batteries in their phone from one or multiple charging devices.
Replaceable module shells are a unique feature of the Ara architecture. They allow users to leverage consumer-grade, full-color 3D printing to aesthetically customize their Ara phone before purchase, and if desired, to replace each module shell any time thereafter.
There are set to be three different sizes of Endo—mini, medium, and large—and obviously the larger the frame, the more can fit onto the device. There will be some limitations on what goes where, though, with each phone having to have a central 'spine' and few 'ribs' to keep things divided. From there it's carte blanche, though, and Google details modules for Wi-Fi, batteries, chargers, displays, speakers, and even a thermal imager.
It also seems like Google is pretty happy for modules to stick out from them in any which angle. So, it shows how sensors—like the pulse oximeter pictured below—could stick out of one end of the phone, while cameras or other sensors might protrude out of the back.
Elsewhere in the document, Google explains that users will be able to order new parts online—probably via Play—working through a design and specification online before ponying up for the technology. If you're so inclined, you can read the 81 page document from the Ara site. [Google]