The 4th generation laser normally comes with a side mounted red laser dot. This works ok but it is not as a good as beam combiner. A beam combiner is a special optic that passes the CO2 laser through the back and reflects red laser light from the front, combining the two beams together. This guarantees the red laser light and CO2 beam travel along the same path off the mirrors and down the lens for precise positioning. The holes are predrilled for easy mounting on the 4th generation laser.