What is the difference between the toner cartridge and the drum unit?

I just replaced the toner cartridge and the LCD continues to display the message "Toner Low" / "Replace Toner" / "Toner Life End" or the Toner LED is still on or flashing.

I still receive the error message "Replace Parts Drum" or "Replace Drum" after I replace the drum unit with a new one.

Replace the drum unit.

Replace the toner cartridge

I don't think I got the full yield from my high-yield toner cartridge but the message "Toner Life End" or "Replace Toner" is displayed on the machine or the LED is on or flashing.

Can I buy any MICR toner?