You can find guidance here:
Residence, remittance basis etc (Self Assessment SA109)
You must fill in boxes 34 and 35 if you’ve put ‘X’(only) in boxes 28, 31 or 32. The Remittance Basis Charge (RBC) is Income Tax, Capital Gains Tax, or a combination of both, charged on unremitted foreign income or gains on the arising basis. When you pay the RBC you must tell us on what income or gains the RBC is chargeable by nominating the appropriate income or gains. You do this by putting the amount of nominated income in box 34 or the nominated foreign gains in box 35. Tell us about your nominated income and gains in the ‘Any other information’ box, box 40. To work out the amount of nominated foreign income, you must convert the income into UK pounds using the exchange rate at the time the income arose.
If you’re not sure, ask your tax adviser. If you’re calculating your own tax, include the Income Tax or Capital Gains Tax elements of the RBC in your total tax amount. If you’re using the ‘Tax calculation summary’ page add either £30,000 or £60,000 to the amount in box 1. Do not forget to show the loss of allowances and AEA in your tax calculation. If you’re liable to pay the RBC you must put a nominated figure of at least £1 in either box 34 or box 35.
You can find further guidance here:
RDRM32320 - Remittance basis: accessing the remittance basis: remittance basis charge - nomination of foreign income and gains: making a nomination