IHTM30102 | Definition and extent of liability (settled property): persons other than trustees
From HM Revenue & Customs · Inheritance Tax Manual
IHTA84/S204 (6) provides that a person liable for tax under IHTA84/S201 other than as trustee of the settlement is liable
You can find examples of the operation of IHTA84/S204 (6) at IHTM30072 which deal with how this subsection works in relation to IHTA84/S199 (1).