Maintenance kits consist of a roller assembly and a counter unit. The roller is impregnated with a fluid that coats the surface of the print drum prior to every print. The melted ink is then jetted onto this surface. The counter tracks the number of pages (pixels) printed and also tracks the life span of the kit (15-24 months depending on model). You can check the percentage life remaining of your maintenance kit from the menu panel of your printer ("Supplies Info" menu). The printer will also display a warning message when the kit is nearing the end of its life. If the roller is low or out of fluid, your printed pages will be missing patches of ink (which will be sticking to the print drum).