In Xcode 6.x, the
Edit -> Paste and
Edit -> Paste and Preserve Formatting menu items are self-evident. What does
Edit -> Special Paste do?
On my system it's disabled most of the times.
It's a fairly obscure and hard to find official documentation covering it feature of the OS X pasteboard being capable of storing several presentations of object data on several pasteboards.
Basically, you can have multiple items in a pasteboard and they are object oriented - so for image if you have copied an image - there is metadata that you could paste - especially when the image itself isn't passable into a plain text document.
When you have a png image in your clipboard, the "special" is renamed:
Paste Image Name
I presume there are all sorts of items you could have "special" aspects of which to paste into an Xcode document in addition to the name of an image, especially if an UTI is defined for that object: