In Delacon's call tracking solution, a CID is a campaign/company ID which is a reference to a single tracking number or pool of tracking numbers. This is used in the tracking code in order to update the existing number on a website with the correct number(s) for call tracking.
The benefit of using a CID in the tracking code rather than the tracking number is that the Delacon number can be changed or numbers added/removed from a pool without the need to change any configuration on the website.
The CID is used in conjunction with your MID (master ID)