This might be helpful for the discussion (from 2019/20 Equipment Book):
ZH4 Downhill speed regulation for 4x4
Compared to Sprinter 906: Carry Over
The code ZH4, Off-road operation, includes a Downhill Speed Regulation (DSR). DSR supports the driver with the Low Range transmission ratio when the vehicle is driving downhill, off-road, and on construction sites. DSR [also] maintains a preset speed on downhill gradients by applying the brakes as required. The speed can be set between about 2.5 mph (4 km/h) and 11 mph (18 km/h) using the brake and accel-erator pedals or the cruise control lever. When the vehicle is faster than 11 mph (18 km/h) off-road, DSR switches to standby mode. DSR remains activated but does not brake automatically. If the vehicle is slower than 11 mph (18 km/h), DSR sets the speed to the previously set speed. DSR switches off automatically when the speed is exceeding 28 mph (45 km/h). The Low Range transmission ratio is engaged, the indicator lamp in the instrument cluster lights up and the vehicle is stationary or not faster than 18 km/h.
1. Activates DSR and stores the current higher speed
2. Activates DSR and stores the current speed
3. Activates DSR and stores the current lower speed
4. Deactivates DSR