This is an Xcode Manipulation API maintained by the Unity iOS team.

The API is bundled with Unity 5.x. The documentation of the version bundled with Unity is provided here http://docs.unity3d.com/ScriptReference/iOS.Xcode.PBXProject.html.

In order to use this API instead of the dll bundled with a particular Unity release, users need to change the namespace of the API to something different to avoid conflicts.

The above caveat applies to Unity 4.6 too, because it uses an older version of the API internally.