In my case that would be the ‘auxiliary’ resistive heater which is part of the heat pump itself; my NIBE system has no mechanism to fire up the DHW cylinder’s immersion element, but it will use its own resistive heating element if it thinks the compressor isn’t able to deliver the heat required (either for DHW or for space heating). I get data on the use of the ‘auxiliary’ heater from the NIBE API and could subtract that from the electricity meter reading and report it separately - but in my case it would almost always report zero since I’ve made configuration changes to try to ensure that heater never kicks in.
I’m happy with that - which was the main reason behind my slightly cryptic question about ‘which immersion’ the other day. My new favourite SEPEMO diagram, as posted to this other thread and linked below, shows two immersion elements - one in the ‘heat pump’ and one in the ‘hot water tank’ - and there’s no metering shown for the ‘hot water tank’ one (although it is inside the H4 boundary line) which supports your view that it’s a separate “system”.
In that case I’ll disconnect the ‘immersion’ feed from my MyHeatPump app (but leave the feed in place so I have the data available if required).