The JV34 can utilize two cartridges per color in 4-color mode. This function allows one cartridge to automatically take over the other supplying cartridge when it runs out of ink.
This frees you from worrying about stop of printing due to out-of-ink during nighttime unattended printing*1.
(*1 … Still, be careful for the data that consumes lots of ink.)
The supplying cartridges are switched at the following timing.
 When “Ink End” is detected during printing
 When “Near End” is detected during ink charging
 When the ink cartridge is removed
 When there is an error on the ink cartridge
 When Ink use-up cleaning function is used
[About Ink use-up cleaning function•••]
During normal cleaning, this function preferentially uses a cartridge with “Near End” or “Ink End” detected, to save ink.
* If the supplying cartridge is removed during printing or cleaning, Using cartridge is changed but the operation stops.