If you don't work and you would get paid for the hours, these will be hours that you have to make up for later. If you do not work and the hours do not have to be paid from you, then the unpaid leave is not deducted from a holiday hours balance and you do not notice anything.
It is true that you can be deployed elsewhere if you would be paid for hours where there is no work at your current workplace.
And if it is because of you that you are unable to work the hours, for example because you cannot make the hours due to school obligations, then you will be paid unpaid leave and you will not notice it.