The Form View is a way to represent end-user configurations of the Case User Interface. Users can define multiple views for one Case UI, as well as designate one as the default view, which displays when the form is rendered. The rendered form only shows one view in each instance. The previous view is replaced if another view is chosen.
Form view switching is always controlled by rules using Show View statements. A very simple example could be using rules to show different views based on the device a form user is using.