The Duet 3 Expansion 1HCL has a number of peripheral inputs and outputs for functions such as sensing motor temperature, controlling a brake and axis endstop. It connects to the Duet 3 CAN-FD bus using RJ11 connectors (the same as the Duet 3 Mainboard 6HC, Duet 3 expansion boards, and the tool distribution board). Multiple 1HCL boards can be daisy-chained on the bus, with power (up to 48V) provided locally. This allows for very large machines to be constructed without a significant wiring burden and signal integrity issues.
The documentation contains more details; a summary of the feature set is here: