counting rules in sap hr

What is the use of a counting rule? 

You define rules for counting attendances and absences. The rules are used to determine the payroll days and hours for an attendance or absence. The payroll days and hours that have been counted are used to control the deduction of quotas. They can be used in Payroll to valuate the absence.
A counting rule can comprise several individual sub-rules. The sub-rules are numbered sequentially. The system runs through the individual sub-rules until it finds a rule that applies.