Both the phono input and the line input refer to two separate stages of the amplifier. The former benefits those with a turntable that requires an external preamp to amplify its weak audio output. On the other hand, the line input is suitable for those with a turntable with a built-in preamp or any other pre-amplified audio device, as it does not increase the signal any further than necessary. Thus, the phono switch will be to do with the activation with this preamp stage in the eventuality that it is needed.