In Reports under Holiday, there is a column 'Earned (for pro-rata purposes).
When an employee books one or more days off these are deducted from their entitlement. Earned (for pro-rata purposes) works as follows.
If you're entitled to 24 days holiday per year you accrue that holiday on the basis of 2 days per month. So for example, if your holiday year starts on the 1st January 2012, and it's the 1st July your staff have "earned" 12 days. If an employee quit their job on the 1st July and had taken 20 days holiday in the period 1st January to 1st July they would actually owe you 8 days back which you would deduct from their final salary.