The flow class means the speed of water flowing through the mixer head which has an impact on the type of stream (wide or narrow) water will flow from the mixer. In the case of a bathtub mixer, the flow class must be higher (B, S) because with a too narrow stream filling a bathtub would take a few hours. In the case of a sink mixer, it can be much lower which also supports water saving (A, Z). As far as the acoustic group is concerned, it is a volume level of a mixer – I means it is silent, and II – louder.