After Apple releases new version of iOS/watchOS/tvOS and you install it, you have some time to downgrade back. Some users are not happy with new iOS or there are apps’ compatibility issues. After releasing of iOS 10, you had time to try it and, if you want, to downgrade back to iOS 9. But time is over. Apple no longer signs iOS 9, including latest version 9.3.5. Actual version is 10.0.2, and it’s older counterpart 10.0.1 is no longer signed too. The same happens with tvOS 9.2.2.
If you still have iOS 9 or 10.0.1, and you need to reset your iDevice, your iDevice will be updated to the latest version of compatible iOS. For example, if you have iPad Air 2 with iOS 9, it will be updated to iOS 10 (10.0.2). If you have old iPad 3, it will install iOS 9.3.5, because iPad 3 does not support iOS 10. It will get iOS 9.3.5, even if it is no longer signed by Apple. It will fully work, but it is unlikely that Apple will release updates for iOS 9, like 9.3.6 or 9.4. I personally own iPod touch 4 and I can use it with latest version of iOS 6.