Application plugins are a mechanism for performing application-wide operations. They may run without a user interface until an application event fires, or they may modify the application menu bar or add application toolbars and toolbar buttons to the AutoCAD Map 3D task pane.

Application menu of the AutoCAD Map 3D task pane in the AutoCAD Map 3D standalone client showing two custom entries from an application plugin.
These plugins are derived from the Autodesk.Map.IM.Forms.ApplicationPlugIn class:
public class MyApplication : Autodesk.Map.IM.Forms.ApplicationPlugIn
{
}