The toner cartridge and drum are different consumables.
The toner cartridge contains powder (toner) and is used to create images.
You need to replace the toner cartridge when the toner is out of stock or the print quality becomes poor due to the deterioration of the toner.
The drum unit transfers the toner to the paper.
The laser printer transfers toner to the roller called "photosensitive" inside the drum unit, then prints the letters and images by fixing the toner onto the paper for heat and pressure.
To maintain good print quality, Brother machine automatically detects the deterioration of the quality of the drum unit. If the Brother printer displays the error message "Replace drum", you need to replace the drum.