SDL Mobile is used with the "context broker" to determine a visitor's device, browser, and operating system aspects. The Context Engine Cartridge (CEC) converts session and request headers into details such as supported screen resolution, OS, and video support using server- and client-side components against the Context Repository. CEC claims are available to use in the Web Application, templates, and other modules that use the ADF such as SmartTarget.
SDL mobile is add on in Tridion 2013SP1 and is integral part of Web8. When a user visits our site the context broker intercepts the request and tries to figure out the information about visitor. It passes this information to SDL mobile which then finds out information such as screen resolution, os, browser, video support etc.