Meaning of the '+' in PowerShell new-object declarations

I'm wondering how to know when to use the '+' in PowerShell object declarations.

Why, for example, is it: $searchFilterC = new-object Microsoft.Exchange.WebServices.Data.SearchFilter+SearchFilterCollection

and not: $searchFilterC = new-object Microsoft.Exchange.WebServices.Data.SearchFilter.SearchFilterCollection

Thanks!

The '+' indicates a nested type, see here for more information.