The services provided by professionals in particular field. For example, a company may charge a certain sum of money for the certain tasks their consultant accomplished for the company who ordered the service.
In the past, when you call an office building or business, you would either be transferred to the department you want to talk to, or given their extension number. This extension is usually a 2-3 digit code that when dialed, transfers you automatically to the particular person or department you want to speak with. The reason why this code works is because it is programmed into a PBX system.