Restrictions on the freedom to dispose of assets: what is a relevant event?
A relevant event is defined in IHTA84/S163 (3). Broadly speaking it is a chargeable transfer where the value of the property (IHTM04030) that is subject to the restriction is brought into account.
In most cases it is the first relevant event that will determine the basis of valuation for property that is subject to restrictions.
You should refer to TG if you are unsure what is the first relevant event.