which industry or SIC code should I fill in for the insurer

SBI code from the Business Register is the starting point

The business class that the insurer wants to know is usually not asked for separately — the insurer asks for a code that already exists: the SBI code that was recorded in the Business Register when the business was registered. This code appears on the extract and is therefore not a new assessment that you need to make yourself. Insurers translate that SBI code internally to their own risk class, with their own premium rate and own terms per class. The answer you enter is therefore literally the code from the extract, not a free description of what the business "actually" does.

It becomes more difficult if the business has multiple activities, or if the main activity has shifted in practice without the Business Register being updated. Insurers then ask about the activity that represents the largest share of turnover or risk, and that is a factual question that only the business itself can answer — not something for which the chain file has a source. In that case, enter the secondary activity with a separate explanation in the free text field, instead of forcing an SBI code that no longer applies. This topic moreover often touches on the same question as why the bank and the insurer do not ask the same question when it concerns the same business: each customer translates the same basic data into their own risk model.

Where the registration requirement comes from

That every business has a recorded business activity that third parties can consult follows from the Business Register Act 2007: the law determines which data of a business are recorded, and the SBI code is the standard component for the nature of the activity. An insurer may, just like a bank that the ultimate beneficial owners of the company requests on the basis of the Money Laundering and Terrorist Financing (Prevention) Act, ask its own additional questions about the exact activity — that customer due diligence is a different legal basis than the registration requirement from the Business Register Act, and that explains why the question is sometimes phrased slightly differently than the Business Register formulates it.