If a trust is liable to IHT or SDLT or SDRT it must register on TRS as a taxable trust: TRSM25030. But it is impossible to do this online as the system only allows the trustee to choose income tax or CGT and otherwise sends you down the non-taxable route. How does the trust fulfil its obligation to register on TRS? Clue: the answer is not to write to Trusts and Taxes
You would register under income tax or capital gains tax depending on which tax the trust is liable for. The guidance at:
Register a trust as a trustee
advises Trusts that are liable for Income Tax or Capital Gains Tax for the first time. Register your trust on or before 5 October in the tax year after the one in which the trust starts to receive any income or has capital gains, and becomes liable for Income Tax or Capital Gains Tax.
For example, if your trust received some interest for the first time in May 2019 (the 2019 to 2020 tax year) and became liable to Income Tax on it, you should have registered on or before 5 October 2020 (in the 2020 to 2021 tax year).