A Switcher (Multiplexer) allows several camera signals to be recorded onto one videotape. To do this it synchronizes the camera signals and marks each one with a code, allowing every camera to be replayed independently, regardless of how many cameras have been recorded.
Many multiplexers also provide the ability to view several cameras simultaneously on one or more monitors. These groups of pictures, when displayed on one monitor, are usually called multi-scene pictures. This is particularly useful when there are a large number of cameras across a site.