Tracking & optimisation · also called gclid, fbclid, ttclid, msclkid, cid
Click ID
A click ID is a unique code an ad platform or tracker attaches to each ad click so that later events can be matched back to that exact click.
When someone clicks an ad, the platform adds a long random-looking code to the web address. Google's is the GCLID, appended by auto-tagging and required for its website conversion tracking. Meta's is fbclid, TikTok's is ttclid, Microsoft's is msclkid. A tracker generates its own as well.
The click ID is what makes arbitrage measurable. One visit passes through several companies: the traffic source, the arbitrageur's lander, the feed. Each knows only its own part. The click ID is the thread tying them together: stored on arrival, passed along as a Sub ID, and quoted when revenue is reported through a postback or Conversions API.
It must be handled exactly. Meta's documentation notes that the fbclid value is case-sensitive and must not be modified, and that it should be sent in a set format (the fbc parameter) with the time it was captured. IDs lost through redirects that strip parameters are a common cause of "missing" conversions. Click IDs are also personal data under laws such as GDPR, so they should be collected and shared only within the consent given.
Think of it like this
It is the tracking number on a parcel. Every depot scans the same number, so the sender can see where the parcel went and whether it arrived.
Related terms
Postback (server-to-server tracking)
A postback is a message sent directly from one company's server to another's to report that a tracked click produced a conversion or revenue.
Tracker (campaign tracking platform)
A tracker is software that records every click an arbitrageur buys, matches it to the revenue it later earns, and reports profit by campaign, ad and keyword.
Sub ID
A sub ID is a custom label a publisher passes to a feed or network with each visit, so reported revenue can be broken down by that label.
Conversions API (CAPI)
The Conversions API is Meta's server-to-server interface for sending conversion events and their values to Meta directly, without relying on the visitor's browser.
UTM parameters
UTM parameters are standard labels added to the end of a link, such as utm_source and utm_campaign, that tell analytics tools where a visitor came from.
Redirect
A redirect automatically forwards a browser from one web address to another, often passing through a tracking link that records the click along the way.
Sources: Google Ads Help: Google Click Identifier (GCLID), Meta for Developers: fbp and fbc parameters (ClickID)