I believe this tells the Form that the user is being forced onto a particular View, and that the View Menu should be hidden, thus preventing the user from changing views.
This URL parameter is added via "View Rules" (sysrule_view table). When you configure a View Rule to take the user to a particular view, it will add this Parameter to the URL, and that, in turn, will cause the View Menu to be hidden from the context menu of the form.
