When requesting leave the amount of hours requested is entered. In some case it says zero hours, which may have the following reasons:
Your schedule says you're not working that day
To determine how many hours of leave should be taken off of your leave allowance, we look at your work schedule. If you schedule says you don't work on a certain day then it also doesn't go off your allowance.
It's a (national) holiday
Leave requests that overlap with (national) holidays will not be taken off your leave allowance. You can find the overview of (national) holidays in your profile (Leave > Holidays).
Your admin has created this setting
Admins can determine for each request whether it should be taken off your leave allowance. When your admin has processed it so that it won't be taken off your leave allowance, it won't cost you any holidays!