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Most Epson cartridges have a chip installed on them; this chip holds information about the ink level in the cartridge.
When the cartridge is empty you'll see a prompt on your screen to replace the cartridge thanks to this chip.
This information has a large margin of error though, as the chip estimates the ink levels. Often the cartridge is not empty at all and you end up throwing out perfectly good ink.
If you reset the chip, using a device like this one, you can continue to use the cartridge until it is truly empty. This can save up to 30% of your ink.