I Version 4.1.32 I cannot select a Product under Print Jobs and assign it to a Printer. I am able to put it it on and when I save and go back to Print Jobs, it show a * under Product Groups and a * under Products.
I also tested the issue with Version 4.1.37 and 4.1.40 and it does the same.
What it prints depends on your printer template. What you did here is mapping a product to a printer so x product will print from y printer. For example you can map drinks to bar and foods to kitchen printer.