When I was in advertising, we used to lay clear acetate sheets over our mechanical board as a way to create variations for our layouts. One acetate layer might have copy and prices for a test market newspaper ad. Another piece of acetate might have prices for a special Sunday-circular ad. Yet another might have copy without prices for the national magazine ads. The artwork and other graphics stayed on the bottom layer and were visible through the acetate layers.
When the mechanical was sent to be printed, the print shop workers flipped the different acetate sheets on or off the board to create the different types of ads. Because a new mechanical didn’t have to be created for each variation, it saved a lot of time and effort.