From Xojo Documentation
Represents an exception that is not necessarily caused by programmer error but rather something from the OS. A good example is trying to write to a read-only file. Checking the writability in advance introduces a 'time of check to time of use' race condition, so the correct thing actually is to just open it.
POSIXErrorDomain POSIX The error domain for error codes coming from POSIX functions in the form of errno values (Mac and Linux).
WindowsErrorDomain Windows The error domain for error codes coming from Windows functions in the form of HRESULT values.
XojoErrorDomain Xojo The error domain for the framework's cross platform error codes.