From Xojo Documentation
Design Property (as String)
| 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.
Uniform Type Indicator used by macOS for opening files. Can only be set in the File Types Set Editor.
Read about UTI on Wikipedia.