Hansgrohe Shower Controls:

See the explanation of the difference between concealed and exposed mixers if you are not sure which you need.

Thermostatic mixers will automatically adjust the temperature to the level you have set, if there is a change of flow in the hot or cold feed. In certain cases, this could occur if a toilet is flushed, a washing machine draws on the hot water or a tap is switched on in the viscinity of the shower. A non-thermostatic mixer would need to be adjusted manually in such a situation. Thermostatic mixers are also potentially safer as they can limit the temperature to a given heat, thus preventing any chance of scolding, therefore being safer mixers for children and the elderly.