The Canon MC-G03 Maintenance Cartridge is a necessary component for the following printers only:
Canon MAXIFY GX3060 MegaTank
Canon MAXIFY GX4060 MegaTank
The Maintenance Cartridge serves as the receptacle for waste ink during cleaning and normal startup subroutines, and is required for operation as once it becomes full the printer will no longer continue to print till its been replaced.
Periodic exchange of the MC-G03 assures the highest quality printing results with your MegaTank printer. Your printer's software will indicate the status of your MC-G01, and for best results Canon recommends it should be exchanged at around 80% capacity.
Canon Support has provided a short video guide on how to replace this maintenance kit.