Hi Puneet,
Taking a quick look thru the 5th edition of the PMBOK I don't find the term "contract management system" anywhere. The first lines of Chapter 12 on Project Procurement Management speak of contract management and change control processes related to procurement management. This refers to the processes used by the organization to manage their contracts/purchase orders. It could be an electronic system, entirely manual, or some combination of both. The point is that there are defined ways (processes) to handle the contracts and keep track of them.
The PMBOK glossary provides this helpful definition: Contract Change Control System - The system used to collect, track, adjudicate, and communicate changes to a contract.
I hope this helps a bit.
Regards,
Tracy