Most users should use Affidavit of Domicile (MPC 485). If you need to save a partially filled-out form, you may choose to use the alternative form:.Your legal and residency status in Massachusetts affects how you file your taxes. Find out what you should file and how to report your income. Legal residence (domicile). To change domicile, a taxpayer must demonstrate that he has taken affirmative steps consistent with his declaration. The IRS provides this residency certification on Form 6166, a letter of U.S. residency certification. A new online form for you to change your state of residency or indicate your income tax withholding election.