If I'm a janitorial supervisor in Kansas City, I can't find out how many other candidates are vying for those jobs. Similarly, if I'm shopping for a janitorial supervisor, I can't tell how many other employers are also in the market. An efficient market promptly reveals the number and quality of players and current supply and demand. Given a locale, time frame, and job title, the assessment service should be able to report the range of pay rates, the number of similar openings in the area, the number of active and passive candidates, and the number of people occupied in this or similar roles in this area. It might look like this.
Every application service is open to competition.
Rarely is only one approach the best for all situations. In some areas, like ranking or matching, there are wildly different approaches that are more suitable for one purpose or another. ASPs publish the inputs and outputs for their services. This level of openness promotes competition without revealing the magic inside the black box. Staffing firms and job boards aggregate application services.