Apple has released the second generation beta of iOS 10.2.1 and macOS 10.12.3. Paying developers can download them now and play on their iPhones, iPads and Macs. As a rule, Betas for participants of the free Apple Seed program appear one or two days later.
What exactly are these?
The beta 2 is to contain performance improvements and bug fixes. Exactly what Apple did not make, so it is unknown which new functions Apple installed in iOS 10.2.1.
The same picture in the beta 2 of macOS Sierra 10.12.3. Again, there is no exact clue what has changed. Apple reports only bug fixes, as well as security and performance improvements. Apple reveals this probably only when publishing the final version. This could take some time. In the previous version of macOS Sierra 10.12.2 Beta phase lasted about three months.
MacOS Sierra 10.12.3 Beta 2 can be downloaded and installed by many developers via the Apple Developer Center or through the update mechanism in the Mac App Store.
The betas are generally not to be installed on productively used systems. In any case, it is a good idea to create a backup before installation – either locally via iTunes (iOS) or iCloud. With macOS, a time-machine backup is available, in order to be able to quickly restore a functioning system in case of any damage.