It's basically gating the sensing.
Normal should be abled to handle most loads and will probably do anything from the Light setting to the Heavy setting.
Heavy will probably force a long, hot main wash and a certain amount of prerinsing.
Good for baked on stuff since that might not be picked up by the sensor during the prewashes.
The normal cycle might thus shorten the main wash wrongfully, thus Heavy is needed.
On the other hand a large amount of easily rinsed off soiling (think whipped cream, fresh coffee, cake residue) might trigger a longer main wash on the normal setting even though you just cleared off the coffee table and the entire load was basically clean after a prerinse.
The light cycle will limit the machine from going overboard.