From Xojo Documentation
| This property was deprecated in version 2010r5.
Please use FolderItem.MacFSRef as a replacement.
Property (As Integer )
| This property is only available on the macOS platform.
For cross-platform compatibility, use ＃If...＃Endif with the Target... specifiers to make sure you will not attempt to use this property on an incompatible platform.
Macintosh volume reference number (Macintosh only).