For what and how can we use ExtensionEndpoint?

[DevTKSS]

Developing my apps I did see some times that an .Extension() can be added and it's naming itself ExtensionEndpoint same for .Validation() used uno Features are e.g. Hosting and Navigation but in no case I seen the second one were Validation added. I did not find information about this and the intellisence or definition lookup did not make it any clearer. Maybe someone can tell me?

[DevTKSS]

@kazo0 seen the chefs app is using a ExtensionsEndpoint at App.xaml.host.cs 🤔 do you maybe have some information about this?