The SA101 is a form that is used as a supplementary page to the main SA100 form when filing a self-assessment tax return in the United Kingdom. The SA101 is used to report specific types of income and gains, such as foreign income and capital gains on foreign assets. The form must be completed in addition to the main SA100 form and submitted to HM Revenue and Customs (HMRC) along with the SA100 as part of a self-assessment tax return.

For foreign income, the form is used to report foreign income such as foreign employment income, foreign pensions and annuities, and income from foreign property and assets. For foreign capital gains, the form is used to report capital gains made on disposals of foreign assets. The form will ask questions about the foreign income and gains and the country that it came from.

It's important to note that if an individual has foreign income or gains and doesn't report it on the SA101, they may be subject to fines and penalties, If you have any questions about the SA101 form or need help with your self-assessment tax return, you may want to consult with an accountant or tax professional.