SA107 form is used for tax purposes in the United Kingdom. It is a self-assessment form for individuals who are required to file a tax return. The form is used to report income and gains from various sources, such as employment, self-employment, and rental income, as well as any tax reliefs and allowances that can be claimed. The form is typically used by individuals who are self-employed, have multiple sources of income, or have complex tax affairs. The form must be completed and submitted to HM Revenue and Customs (HMRC) by the 31st of January following the tax year for which the return is being filed.