The acid descaler DS010 is used only when your combi boiler needs a hard clean due to scale build up. This generally happens when the operator has failed to carry out a CALOUT when requested to by the oven. This can cause a scale buildup and for the oven to stop operating at optimum performance, the steam generator itself may indeed not function at all. If acid descale is required this should be carried out by a qualified engineer.
NOT TO BE CONFUSED WITH CALFREE DESCALER, WHICH IS WHAT IS REQUIRED TO PERFORM A STANDARD CALOUT!!!