Kedhar provided you with a screen capture of an Absence where you can see "Absence Days", "Calendar Days" and "Quota Used".
Absence Days will be the number of days the employee was scheduled to work and was absent.
Calendar Days will be the number of days the employee was absent, regardless of Daily Work Schedule.
Quota Used will be the number of days that will be impacted on the Quota by the Absence.
In Kedhar's example, the 3 Calendar Days equal to 3 Absence Days because the employee was scheduled to work on 12.09.2013, 13.09.2013 and 14.09.2013. If the employee would not have been scheduled on one of these 3 days, the 3 Calendar Days would equal to 2 Absence Days (and still 1 day of Quota Used).