Your boiler is currently working to heat your hot water tank and this status code is just a normal operational message rather than a fault.

domestic hot water demand

What causes this fault?

This status occurs when the external cylinder thermostat or the boiler's internal controls detect that your hot water tank has cooled down and needs reheating. The boiler physically switches its internal valves to direct heat toward the water cylinder rather than your radiators to ensure you have a full supply of hot water for taps and showers.