With this checkbox you can chose whether new lines only apply for new days / nights / etc., or overwrite all days (even those already defined by previous lines).
Let's look at an example to clarify what we mean:
In this screenshot you see a setup, in which the first 4 days cost 100€, the 5. and 6. day cost 90€ and from 7 days each day costs 80€.
Now let's imagine someone books for 8 days and compare the two cases (checkbox on vs. checkbox off)
Checkbox off:
With the checkbox off the system only looks at which line applies, which in this case is the third line (7 - ∞) and calculates all days with this price.
So it also applies backwards.
The calculation will simply be 8*80€=640€
Checkbox on:
With the checkbox on, the system only applies the price changes for the days starting from the given line.
This means that for a booking duration of 8 days the calculation will be:
4*100€ + 2*90€ + 2*80€ = 740€
That is because for each individual day we look at what line its price is defined on.
So the days 1-4 are defined on line 1 (price 100€), the days 5 and 6 are defined on line 2 (price 90€) and the days 7 and 8 are defined on line 3 (price 80€).
So in Summary:
If your price reduction should only apply for future days / nights / etc., set the checkbox to on.
If the price reduction should apply for all days (even those from before when the line applied), leave the checkbox off.