ClipArt is very similar with Group, or say it is a extended version of Group. Besides all the features that a Group has, it enforce all its members to be converted as a static picture in the HTML5 simulation. Converting elements that are "static" in simulation to a ClipArt can reduce the DOM objects in your HTML5 simulation and hence accelerate the loading.
The same as Group, ClipArt is a special element to conjoin multiple elements, and you could not find it from the Elements Panel. After selecting one or more elements you can click the button in the Tools Panel to convert it/them into a ClipArt.
Although ClipArt will be output as static picture in the simulation, you can still edit its member in the editing mode, just like they are within a Group.