clickTag

An ad server has to count every click and then redirect to the landing page. To do that it writes its tracking URL into a JavaScript variable called clickTag when serving the ad, usually as a URL parameter of the iframe. On click the creative opens window.clickTag instead of a hard-coded address. The convention dates back to Flash and is the IAB standard for HTML5.

Not every platform uses it: Google Ads forbids custom click code and makes the whole ad clickable itself, Adform additionally requires a manifest.json, and affiliate networks replace a placeholder in the code. A creative with a wrongly wired click gets rejected or counts no clicks.