The RK 501/S and RK 501/SU differ in that the /SU version has an additional voltage selector switch.
This is used to switch from a voltage of 230 volts, as is common in Germany and many other countries, to 115 volts. This means that the hotplate can also be used in countries with lower voltage values (115 V).
Please note: An adapter may be required for this (e.g. in the USA).