IHTM14502 | Lifetime transfers: the charge to tax: cumulation
From HM Revenue & Customs · Inheritance Tax Manual
When you consider how to apply cumulation, there is a variety of factors for you to take into account. Your approach will depend on whether you are looking at
the chargeable value of the death estate ()
tax directly attributable to a Potentially Exempt Transfer (PET) ()
tax on an immediately chargeable transfer (), or
additional charges on an immediately chargeable transfer arising on death ()