This doesn’t look right, and I don’t know why it works. If you put the name “nnn” in the info.plist, the selector it looks up should be “nnn:userData:error:”. The Swift 4 function signature that translates into this selector would be:

It’s possible that the service lookup tries a number of variant selectors (like KVC does for accessors), for backwards compatibility with older naming conventions, and you were lucky enough to hit on one that works. If you’re interested, it might be worth trying that last function signature to see if that works too.