This is an abstract base class. 

The ModifiedRevision property represents the last version of AutoCAD that modified this element. This property is automatically updated.

Autodesk.AutoCAD.Customization.ElementBase
    Autodesk.AutoCAD.Customization.VersionableElement
        Autodesk.AutoCAD.Customization.CustomizationElement
        Autodesk.AutoCAD.Customization.ToolPanelRef

Public abstract Class VersionableElement
Inherits ElementBase

public abstract class VersionableElement : ElementBase;