While Named User Licensing is associated the license with the user, Shared Device Licensing associates the machine. So, when you launch a named user licensed app, the license is validated against the user name and password. When you launch a shared device licensed app, the license is validated against the machine on which the app is license is installed. This implies that a named user licensed app can be activated on any machine as long as the user name and password are validated. While shared device licensed apps will only activate on machine on which the license is installed.
Read more about Adobe's licensing methods.