Generally speaking, RapaGUI tries to include as few platform-dependent features as possible to allow the same application to run on a wide variety of platforms without any adjustments. There are, however, very few attributes and methods that, although not available on all platforms, are still included because they were considered so important that they have been included although they somewhat contradict RapaGUI's platform-blind GUI toolkit approach. For example, the Window.HideFromTaskBar attribute is only available on Windows and GTK+ since there is no taskbar on AmigaOS and macOS. Another example is Application.OpenConfigWindow which is only available on AmigaOS and compatibles.
Features which are platform-specific are always documented as such. You just need to look into the documentation to see if a certain feature is platform-specific. To minimize platform-specific code, all platform-dependent features can be specified on all platforms supported by RapaGUI as well. RapaGUI will just ignore the attributes and methods then instead of reporting an error message. Thus, you could also set Window.HideFromTaskBar on AmigaOS and macOS. RapaGUI won't report an error message. The call will simply be ignored.