The value of this attribute is an
NSNumber object containing an integer. If missing or 0 or negative, not read only; 1 or more, read only.
- iOS 7.0+
- macOS 10.0+
- Mac Catalyst 13.0+
- tvOS 9.0+
- watchOS 2.0+
Note that this has nothing to do with the file system protection on the file, but instead can affect how the file should be displayed to the user.
OS X v10.3 and earlier string constant is