This is class Autodesk.AutoCAD.Runtime.JavaScriptCallbackAttribute.

Autodesk.AutoCAD.Runtime.IJavaScriptCallable
    Autodesk.AutoCAD.Runtime.JavaScriptCallbackAttribute

Public sealed Class JavaScriptCallbackAttribute
Inherits Attribute
Inherits IJavaScriptCallable

public sealed class JavaScriptCallbackAttribute : Attribute, IJavaScriptCallable;