@scottalanmiller We're targeting all Sandy Bridge, Ivy Bridge, Haswell, and Broadwell Chromebooks. Many users also reported success with Bay Trail, but we don't officially support that just yet. We also hope to support Braswell. Some of the super old Atom Chromebooks and ARM Chromebooks are not supported. To answer your question simply, it's for x86_64 Chromebooks.
Edit: To clarify, an ARM build could happen at some point in the future, but nobody on the team has any ARM Chromebook, plus we're not even sure how we would go about booting it on those devices since there isn't any custom firmware (or built-in SeaBIOS) that we're aware of. We haven't done a ton of research on those models, so it's possible we missed something obvious. To reiterate, it could happen at some point in the future, but not right now.