https://learn.microsoft.com/it-it/dotnet/api/system.windows.markup.ambientattribute?view=netframework-4.8.1
AmbientAttribute Class (System.Windows.Markup) | Microsoft Learn
Specifies that a property or type should be treated as ambient. The ambient concept relates to how XAML processors determine type owners of members.
class systemwindowsmarkupmicrosoftlearn