The Duet board features two different ports for the extruders, thermistors and heaters. Additionally, the extruder assembly wiring features different connectors for all of these components. On top of that, you can configure the software settings of each of the tools in order to use particular heaters or extruders. This all combined allows you to create a large number of configurations for the extruder wiring. This standard is meant to explain and standardize that wiring.