A Key Number is a tracking device used in various forms of advertising. You often see it utilized in classified or display ads in newspapers and magazines. The advertisement will show a specific phone number, a specific phone extension at an 800 number, or a mailing address with a specific department number.
Each ad that the business places has a different key number depending on what specific publication they are advertising in. When the prospect inquiries come in, the business can then track which advertising is having the most pull. This will help them decide where to place advertising in the future. This also allows them to test different advertising layouts and artwork.
When advertising on the internet you can also utilize this same basic method. If you buy an advertisement on a particular site, you can have your link go to a specific entry page on your web site. You could then check your server logs to see which entry page is getting the most hits. This will tell you which ad has been getting the most pull.
You could also advertise with an email link instead of a link to your web site. Likewise you could utilize different email addresses as key numbers to see which ad is getting the most response.
You could even get more advanced readings by utilizing scripting technologies such as PHP or email features such as auto-responders.