The SA109 form is a supplementary page to the Self Assessment tax return used to record residence and domicile status. You can normally submit supplementary details as part of your tax return if you submit online, but to provide residence or remittance basis information you’ll either need to buy special software to do it online, or use the SA109 paper-based form.
You should complete an SA109 and include it with your tax return if you’re not a UK resident, or if you are a UK resident but you:
Are eligible for overseas workday relief (OWR). This is a type of tax exemption which treats UK employment income earned abroad as if it were a foreign source of income.
Became resident in the UK during the tax year that you’re reporting
Are due split-year treatment (where you pay income tax as a UK resident on income which you earned in the ‘UK part’ of the year, and as a non-resident for the ‘overseas part’ of the year)
Are domiciled outside of the UK
Want to use the remittance basis on foreign income or capital gains (where you only pay UK tax on the UK part of your income and gains, and only pay it on the foreign part of your income and gains if you bring them in to the UK)