is “Series” part of the Pro plugin? I think it is. This is only a guess but this post type is probably uses for children of “events” and this is done though code, the only way that a post type can be made a child of another post type, and for the most part this post type is hidden in the admin because it is not meant to be edited directly but only edited through the “event” parent.
I created my own plugin that does something similar with post types. There is no way, except by exploring the code, that anyone would know the child post type exists.
