SpotLight is a projection texturing system which allows you to prepare your source texture directly in ZBrush, then paint your model with it in 3D.
You first need to load your textures using the Texture palette or Light Box. You can then use SpotLight to change their scale, rotation and position; nudge them to match any sculpt; clone parts of the texture; fill colors; change the hue to match another texture’s hue and more.
There is a tremendous amount of functionality packed in a simple interface. Your painting productivity will increase more than you can imagine!