Renting SIXT share cars using the hvv switch app is only possible within the SIXT share service region in Hamburg. However, you are permitted to drive outside of this service region and even abroad. A detailed list of countries where you are permitted to drive can be found in the SIXT General Terms and Conditions.
You can only end the rental in the same country in which the vehicle was originally rented, however. But this can also be outside Hamburg, within a SIXT share service region in another city, or at a SIXT share station.