A caddy (also called a grip) is a collection of transparent UI elements that are shown in the canvas. 3ds Max currently uses the caddy for allowing the user to modify editable polygon mesh surface, but it can be used wherever a plug-in requires convenient in-canvas controls.

A caddy can be implemented by a plug-in by overriding the IBaseGrip interface, and enabling it using the caddy manager (IGripManager) which can be retrieved using the global function GetIGripManager().

There is sample code for caddies in: maxsdk\samples\mesh\editablepoly\polygrips.h and maxsdk\samples\mesh\editablepoly\polygrips.cpp