Some colour laser-class printers use a rotating belt called a “transfer belt unit” to further colour registration accuracy. It is a rotating belt that is typically large which carries the mixture of different colours of toner particles. The cartridges will then precisely apply each toner layer to the belt which will then apply the combined layers uniformly onto the paper. You might need the belt unit to be replaced after some time of use. A belt unit is usually located under the toner and drum.