While this is an engine, it functions as more of a photo editing tool. As the name suggests, this tool allows you to clone an existing part of your canvas and apply it to a different part. The unique settings that will control the clone features are in the Painting Mode.
Press Ctrl + Click on the canvas to select a source point. This source point will be used to copy from. When you start painting on the canvas, the brush stroke will look at the source to get its brush information. As the instructions say on the tool, you can clone from a different layer by selecting the layer and pressing Ctrl + Alt + Click. Outside of photo retouching, I haven’t found much use for this.