The rules on accountability are found in IHTA84/S216.
In general, the accountable person is someone who is liable for any tax (IHTM30011) on the assets concerned. But not all those who are liable for tax are accountable and, very exceptionally, a person who is not liable may be required to deliver an account.
The relationship between accountability and liability was discussed by Walton J in CIR v Stype Trustees (Jersey) Ltd  STC 394, CTT Cases No 27.
Different rules may apply in some circumstances to
- transfers on death (IHTM10811)
- transfers in respect of a qualifying interest in possession in settled property (IHTM10831)
- lifetime transfers that are chargeable when made (IHTM10822)
- failed potentially exempt transfers (PETs) (IHTM10821)
- gifts with reservation (GWRs) (IHTM10824)
- relevant property settlements (IHTM10836).