If you immigrated to or emigrated from the Netherlands during the year, you file an M-form (M-aangifte) for that migration year instead of the regular return. Because wage tax is withheld as if you worked the full year, a part-year stay very often produces a refund. The M-form can be filed online in Mijn Belastingdienst (no more paper booklet), and the deadline in your aangiftebrief is often 1 July. For every year you were a resident the FULL year, you can file the regular return with Taxmo.
The M-form covers only the year you arrived in or left the Netherlands. It splits the year into a resident and a non-resident period, each with its own rules. For all other years (full-year resident) you file the regular P-style return, which is what Taxmo files directly to the Belastingdienst.
Your employer withheld wage tax using annual tables, as if you earned that salary all year. If you only worked part of the year in the Netherlands, the withholding is usually too high, and tax credits are recalculated for your resident period. Refunds of several hundred to a few thousand euros are common for mid-year arrivals and departures.
Since tax year 2020 the M-aangifte can be completed online in Mijn Belastingdienst with your DigiD; the old paper booklet is no longer needed. Taxmo does not file the M-form itself: our direct filing covers regular full-year resident returns. Use Taxmo for your full years in the Netherlands (also retroactively, up to five years back for a refund), and Mijn Belastingdienst for the migration year.