An AnnDesigner derived class controls the user interface involved in drawing, editing or running an object. This class provides the basic functionality common to all designers: such as hooking into a container, clipping the mouse cursor, etc.
You must assign an AnnContainer object to the Container member and then, based on the exact designer functionality, it will hook into the various pointer (mouse or touch) events to draw a new, edit an existing (by moving, resizing, etc.) or running (if the container is in run user mode) an AnnObject.
Refer to IAnnAutomationControl for an example on how to use LEADTOOLS annotations with your own controls.