The SA100 is a form used in the United Kingdom to file a self-assessment tax return. The form is used by individuals who are required to file a tax return, including self-employed individuals, landlords, and those with certain types of income such as foreign income, capital gains, and income from savings and investments.

The SA100 form is the main form used to file a self-assessment tax return, and it includes sections for an individual to report their income, expenses, and any tax reliefs or exemptions they are eligible for. The form must be completed in full and submitted to HM Revenue and Customs (HMRC) by the deadline, which is usually the 31st of January following the end of the tax year.

It's possible to file the tax return online, and HMRC provides an online service and guidance to assist with the process. If you have any questions about the SA100 form or need help with your self-assessment tax return, you may want to consult with an accountant or tax professional.