It is up to your bank to explain why it is asking you for these data. It may need the information to meet obligations arising from the know-your-customer principle under the  Wwft and the Wft, or it may need certain data, such as your citizen service number (BSN), to comply with tax laws. If you refuse to provide them, your bank may be unable to meet its legal obligations. It could decide to terminate its relationship with you as a result.