Working with the UI Source - Unity 3D UI Essentials - page 230

What is in the solution?
Now you have the code on your machine, let's delve into what we have.
Open the folder where the Unity UI code has been copied to on your machine and
open the UISystem.sln file by double clicking it and that will open your code editor
of choice, such as MonoDevelop , as shown here:
You will see three projects in the Unity UI solution:
• UnityEditor.UI : This contains all the Unity Property Drawers ( http://docs.
unity3d.com/Manual/editor-PropertyDrawers.html ) and Custom Editor
windows ( http://docs.unity3d.com/Manual/editor-CustomEditors.
html ) for all the UI components. If you want to extend or add new Editor
features, this is the place to add them.
• UnityEngine.UI : This contains the core UI classes and components, with
everything from the Event System to Images and Toggles plus everything in
between. If you are adding new controls or fixing existing controls this is the
place to be.
• UnityEngine.UI-Editor : This is the editor version of the previous UI code.