The MAXREFDES100# is the featured board that has all the sensors included. It is the platform the supports the measurement of motion, precision skin temperature, biopotential measurements (ECG, EMG, EEG), in addition to photo plethysmography measurements.
The MAXREFDES100HDK# is the (included) companion board, which is a programming adapter that provides driverless drag-and-drop programming for the firmware updates as well as a virtual UART interface and CMSIS-DAP compatible debugger.