Power ON/OFF connects the MF/HF radio to its power supply, so unless or until it is switched on, the radio is useless. For some equipment there is a dedicated push button control, but for more portable equipment it is combined with the volume control.

Volume makes the level of sound higher or lower from the loudspeaker.

Practise on a MF/HF radio simulator

The MF/HF is turned on by a single press on the ON/OFF/Volume button. The MF/HF is turned off by pressing the ON/OFF/Volume button for 4 seconds. This is always indicated by a countdown window in the information display, except if the radio is powered-down in distress mode. Any connected devices (Alarm Panel, Handset, Control Units) will be operational only if the MF/HF is powered. The start-up display is the last used mode.
Note: The equipment should always be switched on while at sea in order to maintain continuous DSC watch.