While Named User Licensing associates 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 a machine on which the license is installed.
Read more about Adobe's licensing methods.