The professional term you are talking about is called "point-breaking bag".

It is cut with a bag cutting machine. You need to change the cutting knife to a point-breaking knife.

The dotted lines are cut with a point-breaking knife. It reminds me very much of the single-sided thinning scissors used by barbers. There is a knife specially designed to cut dotted lines. Replace the cutter in your bag making machine with a breakpoint knife, which is available at bag making machine accessories stores.