Start with the purpose of the twin. A planning team may need to explore a site and explain a proposed layout. An operations team may need a spatial view of information about the place. A presentation team may need to walk stakeholders through a building or city. That purpose determines what the 3D environment should show and how people should interact with it.
The brief should also distinguish the physical place from the information shown about it. Identify the available reference material, required level of detail, intended users and display setting. If live updates or connections to existing systems are needed, define the source, access and update requirements during scoping. The required integrations and operating arrangements belong in the agreed project scope.