A click ID is a unique value generated for every click on a tracking link or ad. It travels with the user to the landing page, is stored by the advertiser, and is sent back when a conversion happens, so the tracker can match the conversion to the exact click, affiliate, campaign, and sub-ID that produced it.
How it works
The tracking system creates the ID at click time and appends it to the destination URL as a parameter. The advertiser's site reads the parameter and saves it, typically in a first-party cookie, session, or database record linked to the lead or account. On conversion, the saved ID is returned through a postback or API call.
Ad platforms use the same idea. Google Ads auto-tagging appends a gclid parameter to landing page URLs so conversions can be tied to ad clicks, including sales that close later through offline conversion imports.
Example
Illustrative URLs:
Tracking link: https://track.example.com/c/45?sub1=email
Landing URL: https://brand.example.com/?click_id=7f3a9c21e8
Postback: https://track.example.com/postback?click_id=7f3a9c21e8&status=approved
Here 7f3a9c21e8 is the click ID. Without it, the postback has no way to say which click converted.
Why it matters
Click IDs are the backbone of S2S tracking. For brands, capturing and storing the ID is one of the most important steps in any tracking integration; losing it in a redirect or form submission means unattributed conversions. For affiliates, the click ID connects network conversions to your own tracker or ad account. Affiliates who buy search traffic often store both the network click ID and the gclid, so they can upload conversions back to Google Ads. See Google Ads offline conversions for affiliates.
Related: postback URL, sub-ID, Conversions API.