Every time a laser imaging device prints a page, a tiny amount of toner is left on the drum which is scraped off and deposited into waste toner bottle. Eventually the waste toner bottle fills up and needs to be replaced. Machines will usually stop printing when the waste toner bottle fills up to avoid overflows, and will refuse to print until the bottle is replaced.
The CLP-W504 waste toner bottle is a critical part of the colour printing process. Every time a page is printed, a tiny amount of toner is left on the drum, which is wiped off with a blade and diverted to the waste toner bottle. Over time, the bottle becomes full and needs to be replaced. Usually your printer will stop printing pages until you replace the waste toner bottle – this is to prevent an overflow of waste toner into your device. A wise choice is to have a spare on hand to minimize downtime.
Surprisingly these waste toner bottles like to be replaced and not simply cleaned out when full. Most printers give a warning this is about to happen however most users ignore this warning. Unsurprisingly the cheapest consumable then becomes the most important.