Thinking more about the use-cases for "bitness" (see #105), it seems desirable to not only know platform bitness, but also the app bitness. For example, if you run an app store webpage, or a support page for an app, you can know if a user has downloaded the most optimized binary, or is in some other "32bit app on 64bit OS" bucket (aka "WOW64").

These 2 hints (UA-App-Bitness? / UA-Client-Bitness? & UA-Platform-Bitness) would be high-entropy hints and not available by default.