The Duet 3 Expansion 1XD is a CAN-FD connected expansion board for the Duet 3 Mainboard that provides a connection to an external stepper driver and associated peripherals. It has standard Duet 3 RJ11 connectors for daisy-chaining the CAN-FD bus and accepts up to 48V to allow sharing power supplies with stepper drivers up to 48V

The documentation contains more details, a summary of the feature set is here:

• ARM Cortex M0+ processor running at 48MHz
• 5V differential Step, Dir and Enable outputs for an external stepper driver (can be used in differential or single-ended mode)
• Two medium-current (2A max recommended) outputs at VIN voltage with PWM capability and built-in flyback diodes
• One 5V-level output with PWM capability (3mA max)
• Three digital inputs with permanent 27K pull-up resistors, protected against over-voltage.
• One thermistor input (reduced accuracy compared to other Duet 3 boards, so PT1000 sensors not recommended).
• RJ11 CAN In and CAN Out connectors to connect to the Duet 3 CAN-FD bus
• VIN: 12V-48V