RLD uses an enum called GizmoDimension to differentiate between 2D and 3D entities. The enum resides inside the Assets\Runtime Level Design\Scripts\Gizmos\Core Data Types\GizmoDimension.cs file and is defined like so:

public enum GizmoDimension
    None = 0,

This is useful when differentiating between 2D and 3D handles. Some gizmo handles are 3D and others 2D. For example, a move gizmo shows only 3D handles when working in 3D mode. But when you hold down LSHIFT the gizmo enters 2D mode and all the handles you can see on the screen are 2D.