https://learn.microsoft.com/en-us/uwp/api/windows.ui.xaml.markup.fullxamlmetadataproviderattribute?view=winrt-28000&viewFallbackFrom=winrt-10240
FullXamlMetadataProviderAttribute Class (Windows.UI.Xaml.Markup) - Windows apps | Microsoft Learn
Indicates that the XAML metadata in a component library is complete.
classwindowsuixamlmarkup