If the combination heater and the domestic hot water tank are sold under two different model identifiers, then the domestic hot water tank is considered as a hot water storage tank and the two components shall be labelled separately. They also need to be tested as individual products and meet the minimum requirements set for them. For solid fuel combination boilers, the water heating efficiency is not regulated.
The label of a combination boiler should include the symbol for the water heating function, see Annex III of Regulation (EU)2015/1187. Hot water storage tanks sold under different model identifiers and combined with these 32 heaters are regulated under Regulations (EU) 814/2013 and 812/2013. They should meet the minimum requirements and be labelled separate accordingly.
Disclaimer: Please note that the European Commission cannot provide a legally binding interpretation of the EU legislation, as this is the sole competence of the European Court of Justice. Any remarks from the European Commission services are without prejudice to the position the Commission might take should related cases arise in a procedure before the Court of Justice.