The nighttime adjustment is applied on top of the upwind/downwind adjustment and allows the router to consider that you sail the boat slower/with less sail at night for safety reasons. It is a percentage factor, so if you use the value 70%, this tells the router you expect to sail 30% slower than the base polar you have set. This value will be added in addition to any other polar speed adjustments you have applied.
Some other relevant information relates to the definition of day/night:
"There are different definitions for sunrise/set: 'official', 'civil', 'nautical', and 'astronomical'. Some references if you like:
The nautical definition is used for the router, which should correspond to when you have enough daylight to use for navigation.
If you use the AI Polars feature, please set your nighttime adjustment to 100%, as the machine learning algorithm already accounts for nighttime sailing and automatically applies it to your routing.