The fee structure is both a filter against superfluous submissions and a revenue generator. Typically, the fee covers an annual subscription for one webpage, which will automatically be catalogued on a regular basis. However, some companies are experimenting with non-subscription based fee structures where purchased listings are displayed permanently. A per-click fee may also apply. Each search engine is different. Some sites allow only paid inclusion, although these have had little success. More frequently, many search engines, like Yahoo!, mix paid inclusion (per-page and per-click fee) with results from web crawling. Others, like Google (and as of 2006, Ask.com), do not let webmasters pay to be in their search engine listing (advertisements are shown separately and labeled as such).
In the parlance of digital marketing, advertisers are commonly referred to as sources, while members of the targeted ads are commonly called receivers. Sources frequently target highly specific, well-defined receivers. For example, after extending the late-night hours of many of its locations, McDonald's needed to get the word out. It targeted shift workers and travelers with digital ads, because the company knew that these people made up a large segment of its late night business. McDonald's encouraged them to download a new Restaurant Finder app, targeting them with ads placed at ATMs and gas stations, as well as on websites that it new its customers frequented at night.