Individuals and organisations which receive income or gains from other countries, typically pensions or royalties, may have overseas tax deducted from those payments unless they can show they qualify for relief under a double taxation agreement (DTA).
Many DTAs require the taxpayer to prove they are resident for tax purposes in the UK before the payment can be made without deduction of tax, or in some cases with a lower rate of tax deducted. This proof is given as a certificate of residence, which in general must be issued by HMRC.