CIS, CISS, Continuous Ink System AND Continuous Ink Supply Systems are terms used interchangeably. It is the means of supplying a continuous flow of ink to the cartridge of the printer without the need to change cartridges. These systems have an ink supply tube which connects an ink supply or reservoir to the cartridges in the printer. A Large external ink reservoirs continually supply the cartridges with ink through tubes. The cartridges remain in place permanently in the printer. The ink reservoirs are independently separated by colors that can be easily refilled tank anytime. It works efficiently in almost all brands and model of inkjet printers. Since ink is drawn from the reservoir to the cartridge you will need to replace ink in the reservoir, not in the cartridge.
Most, if not all CISS come with a cartridge. In our opinion, that is precisely what you want because many cartridges have a chip in them to tell us when the ink is low, that chip is going to need to be updated or replaced for your CISS to work properly.