what do I fill in if I don't know the answer to a sustainability question

Leaving it unanswered is the correct answer, filling in as an assumption is not

If you don't know the answer to a sustainability question, you leave that question open and mark it as "to be determined" instead of filling in a guess. This also applies if you suspect that the answer is probably "yes" or "no". A suspicion is not a source, and a filled-in answer without a source causes more damage in a later audit than an open question with a clear explanation.

For questions derived from EFRAG's VSME-basic module, they often concern figures that your supply chain file doesn't yet have: energy consumption per location, scope 3 emissions from a specific supplier, or the share of revenue that falls under a taxonomy activity. If that figure hasn't been documented anywhere, "to be determined" is factually correct. The distinction with a question you can answer from basic data — such as the establishment address of the company or the number of employees — is that the latter category is already in the Trade Register or your own administration. An emissions figure that has never been measured doesn't exist anywhere, and that difference determines whether "to be determined" is the correct status or whether there's simply a source missing that you can still provide.

Why an estimate is not the same as an answer

The CSRD (Directive (EU) 2022/2464) requires large enterprises to substantiate reported data, and this obligation flows through to the questionnaires they send to their suppliers. A customer who incorporates a figure from a supplier in their own reporting must be able to account for that figure. An estimated or assumed answer on your part without an underlying document thus puts not only your own answer on shaky ground, but also your customer's reporting. This is why a status such as "to be determined" — possibly with a brief explanation of what is missing — is a more complete and honest answer than a filled-in value that no one can verify.