This custom attribute class is used to mark a type so that an instance of that type will be instantiated for each document open/opened in AutoCAD. 

An application may designate as many types as desired with this attribute. When the application is loaded into AutoCAD, a new instance of the type will be instantiated for each document currently open and a reference to the document, and a new instance will be instantiated for each document opened thereafter. When a document is closed, the associated type instance will be disposed if it derives from IDisposable. 

The type associated with a PerDocumentClass attribute must provide either a public constructor that takes a Document argument, or a public static Create method that takes a Document argument and returns an instance of the type. If the Create method exists, it will be used, otherwise the constructor will be used. The Document that the type instance is being created for will be passed as the Document argument so that the type instance knows which Document it is associated with.

Autodesk.AutoCAD.Runtime.PerDocumentClassAttribute

Public sealed Class PerDocumentClassAttribute
Inherits Attribute

public sealed class PerDocumentClassAttribute : Attribute;