https://learn.microsoft.com/it-it/dotnet/api/system.windows.markup.xamldeferloadattribute?view=netframework-4.8.1
XamlDeferLoadAttribute Class (System.Windows.Markup) | Microsoft Learn
Indicates that a class or property has a deferred load usage for XAML (such as a template behavior), and reports the class that enables the deferring behavior...
class systemwindowsmarkupmicrosoftlearn