A keycode is a sequence of letters, numbers, or both, that appears on a response device (such as a magazine response card or a catalog order form) to identify the campaign and the direct mail list used for the mailing. The code SP06AU4, for instance, might indicate the second spring 2006 mailing to a particular list of car (auto) owners.
Using keycodes, a marketer can measure and compare reponses to specific mailings and lists. Keycoding is an essential tool for testing and evaluation of direct mail campaigns.