The quarter sheet shipping label can be used in lieu of the 6 up ML-0600 shipping label. Maybe you have a big logo, or maybe the customers that you’re shipping to have huge address lines, and it’s just too crowded on the ML-0600.
There’s plenty of breathing room on the ML-0400, it’s dimensions are 4-1/4″ X 5-1/2″, and it’s on an 8-1/2″ X 11″ sheet. Just be aware that the labels do not have any margins. The label goes all the way to the edge of the page. You might be designing a logo that goes too near the edge of the page without paying attention to the physical limitations of the margins of the printer.