If you had savings or investments during your migration year, the box 3 savings-and-investments basis only counts for the period you were a domestic taxpayer. The valuation date for the calculation stays 1 January, even if you moved later in the year or left earlier. Taxmo files the M-form with box 3 for €159 incl. VAT (Plus).
If you lived in the Netherlands for part of the year, your box 3 basis reflects only the value of your assets and debts during that domestic period, not the whole year. Savings or investments you already held abroad before moving stay outside the Dutch basis until your migration date.
You are still entitled to the tax-free wealth allowance in a migration year. With a fiscal partner you may only divide the basis jointly if you meet the conditions for that; otherwise each partner reports their own assets separately.
The wizard asks about your assets and debts during your Dutch period and calculates the basis, the tax-free allowance and the tax due on it. You see the result before you pay.