If you had a fiscal partner during your migration year, you usually file the M-form together. A fiscal partner is included in Taxmo's Standard price of €129 incl. VAT, at no extra cost. The harder part is the box 3 savings-and-investments basis, which you may only divide under specific conditions tied to your periods as a taxpayer.
The Belastingdienst allows you to divide the savings-and-investments basis only if you were both a qualifying foreign taxpayer for the whole tax year, or both a domestic taxpayer for part of the year with exactly the same start and end date. Meeting neither condition means you may not divide it.
If your partner moved on a different date than you, your periods as a domestic taxpayer are not identical, so the division rule does not apply. Each partner then reports their own assets separately instead of a shared basis.
The wizard asks about your fiscal partner's situation and applies the division rule when the conditions allow it. If your situation is too specific, for example a partner who lived in Belgium, Suriname or Aruba, the screening questions tell you before you start.