Reddit's click ID, as a plain number
rdt_cid carries Reddit's click ID. Reddit's help center tells advertisers to open an ad's destination URL to "see your click ID, passed as a value for rdt_cid in the URL." Reddit's own macro table lists the click ID as an integer, with the example value 3001238223506745143. Most ad click IDs are long runs of letters and symbols. Reddit's is all digits.
It belongs with fbclid and gclid: a value tied to one ad click, which the advertiser uses to match a later signup or sale back to that click. Reddit notes that a conversion can only be recorded if the advertiser has installed the Reddit Pixel or the Conversion API.
Why it goes missing
Advertisers who route Reddit clicks through Campaign Manager 360, Google's ad server, have an extra step. Reddit tells them to add &rdt_cid=%prdt_cid=!; to the landing page URL suffix in Campaign Manager 360. Without that step, Reddit warns, "Reddit's click ID won't be attached to the destination URL for any Reddit ad", which can make conversion numbers on the Reddit Ads dashboard inaccurate. Reddit's test-link guide gives the same pointer: if the destination URL shows no rdt_cid value, set up third-party measurement.
Other names for the same number
App measurement tools receive the same value under their own parameter names. In Reddit's setup tables, the {{CLICK_ID}} macro fills reddit_click_id for Adjust, clickid for AppsFlyer, ~click_id for Branch, click_id for Kochava and cl for Singular. A tracking link with any of those carries Reddit's click ID under a different label.
Sharing a link that has it
Nothing on the landing page depends on it. If you copy a link after tapping a Reddit ad and send it on, you pass along a value tied to your click, so share the version without it. On the advertiser's side, a missing rdt_cid is a conversion Reddit cannot connect to its ad. See the link decoder hub for other ad networks' click IDs.