http://scn.sap.com/community/erp/hcm/blog/2013/05/01/useful-documents-for-configuration-and-pcr
Hi,
Venkatesh is correct.
When you standard pcr's are not fulfilling your requirement, then we go for custom pcr. Even, it is better to achieve first by configuration then go for custom pcr (examples: absence valuation, time wage type selection using T510S etc).
You can also view the link mentioned above. You can also check wiki pages on this topic.
regards,
Bala.