Overlay is simply another word for stencil – the piece of plastic out of which the design is cut.   Simple stencils will have the entire design all on one overlay/stencil; it’s elements are separated by small gaps or spaces.   If you wish to have a more realistic looking design, a design without gaps or spaces, choose a multiple overlay stencil design.  Multiple overlay stencils will place parts of the design on one overlay and other parts of the design on a second overlay.   Multiple overlay designs look more realistic because there is no space or gaps between the design elements in the finished design.  The design is continuous.  The stenciling techniques used with the one overlay designs are the same as those used with multiple overlay designs.