After just two days in developer's hands, Apple has made the betas for iOS 10.3 and macOS 10.12.4 Sierra available for all registered in the public testing program.
The iOS 10.3 update will update the file structure on a installed device to Apple's new APFS, that was first revealed at the 2016 WWDC. Other changes include a new "Find My AirPods" ability, additions to Siri, CarPlay improvements, and some changes to Maps.
APFS, the Apple File System, is "optimized for Flash/SSD storage, and engineered with encryption as a primary feature," according to an entry in the WWDC 2016 schedule. In official documentation, Apple adds that it uses a "unique copy-on-write design" with I/O coalescing, meant to optimize performance while staying reliable.
Also added to iOS 10.3 is the ability for a user to "Find My AirPods." Opening the Find My iPhone app, users who have previously paired AirPods with an iOS or macOS device enrolled with iCloud will find a new AirPods option under the "My Devices" section. Unless the headphones are in use, Find My iPhone will show the headphone's last known position, as well as information when the positioning data was gathered.
The biggest feature added to macOS 10.12.4 appears to be the implementation of NightShift.