If I remember correctly, it use to be possible to add external object placeholders to a NIB in interface builder. This was used to add reference to existing instances of objects that the NIB doesn't create but need to be referenced by other objects in Interface Builder. Similar to "File's owner" and Application objects.
I also found some mention in older SO questions of this technique, for example here
I am using Xcode 5.0.1 on Mavericks and there is no trace of these "External Objects" in interface builder. It seems that it is not possible to add anything to the placeholders anymore. I went through Xcode's release notes since Xcode 4.0 and there is no mention of this being removed (although there are few mentions of dragging placeholder, whatever that means).
So, any idea if this was intentionally removed? A bug? Is there any workaround or replacement for this feature?