The IRS doesn't recognize registered domestic partnerships (RDPs) as marriages. Because of this, RDPs must file their federal and California tax returns using two separate accounts.
If this applies to you, RDP couples must file a federal return as Single or Head of Household.
Then, to file a California return, you'll need to create a new account and fill out a "mock" federal return as if you were filing your federal return as Married Filing Jointly, Married Filing Separately, or Qualifying Widow(er), if applicable, to correctly calculate federal adjusted gross income and other values for your California tax return. You should NOT file this "mock" federal return with the IRS.