Hi Shing Hsu,
Box 35 is for capital gains not remitted to the UK.
RBC means Remittance Based Charge, which is £30,000 or £60,000.
The remittance based charge only applies, if you have been resident in the UK for more than 7 of the last 9 years and is added into the tax calculation.
If you are resident and not domiciled for less than 7 years, RBC does not apply.
Box 36 is for adjusting the payments on account POSAs) that are due for the follwing year.
You can find guidance on payments on account here:
Understand your Self Assessment tax bill
Thank you