according to the specs I just read its just a dimmer with a Pico on the front of it instead of the paddle. Still uses picos in multi-location applications.

edit: well, I see what your talking about in the back of the book.... quite strange way to address the three way circuit. I'm not sure how that works seeing the three way traveler get capped on the Caseta and the load is jumped at the accessory. Looks like the three way switch is just sending an on trigger to the Caseta maybe?