++something++
is a traversal namespace, so it's not a resource per se, ++resource++
is instead another namespace. Namespaces look up custom namespace handlers, which are given the rest of the item name.
In this case the widget
namespace handler is given the name of the widget, forms.widgets.audio_artifact
, and it will use this to look up that widget on the add form.
@@download
is a view on the widget, that enables you to download the file in the widget. For images there is also a view that allows you to get thumbnails etc.