Overtime balance and leave balance impact each other for employees with a fixed contract (this isn't relevant for on-call contracts).
When someone with a fixed contract requests a holiday for a certain period, and you assigning 0 hours from their holiday balance to it, they will use their overtime balance for the holiday (tijd-voor-tijd) automatically.
For example when someone requests a holiday for 7 days, and they are supposed to work 38 hours that week, if you assign 0 hours from their leave balance to that holiday, their overtime balance will go down by -38 hours.
If you assign 20 hours to that holiday, their overtime balance will go down -18 hours. Since 38 hours work minus 20 holiday hours leaves 18 overtime hours.
If you assign 40 hours to that holiday, their leave balance will go down with 40 hours, their overtime balance won't be touched.
Example
This employee took a holiday from 8 Jan - 18 Jan. So 11 days. They got 60:05 hours assigned to the holiday.
This is reflected like this in their hours. It's calculated as follows:
This employee normally works 40 hours per week. 40 hours divided by 7 days = 5:42 hours per day
11 days times 5:42 hours = 62:51 hours they are supposed to work over 11 days.
Their holiday takes 60:05 hours from their leave balance.
This leaves us with 62:51 minus 60:05 = 2:46 hours of overtime that need to be taken from their overtime balance.
2:46 hours of overtime are spread over the 11 holiday days, which means -0:15 of overtime per day, which explains the -0:15 of overtime for each day of holiday:
