The ink status monitor shows how much ink is contained in the cartridges. The status monitor updates itself during printing via information being passed to it from the chips on the cartridges.

A cartridge’s print life usually ends when the status monitor believes the cartridge to be empty, and the 'empty' cartridge must be replaced in order for printing to continue.

However, Jet Tec Extralife cartridges use 'smart' chip technology: When enabled, the chip's patented program activates the Extralife and displays it in the 'status monitor'*, thus allowing access to extra ink from the Jet Tec cartridge.
NO OTHER CARTRIDGE RANGE DOES THIS.
*Some Extralife cartridges may not display Extralife in the status monitor.