The system version in effect for the archive.
- iOS 2.0+
- macOS 10.0+
- Mac Catalyst 13.0+
- tvOS 9.0+
- watchOS 2.0+
During encoding, the current version. During decoding, the version that was in effect when the data was encoded.
Subclasses that implement decoding must override this property to return the system version of the data being decoded.