Every deliverable on a project has a purpose. It creates an outcome for the client or it provides an outcome required for another task within the project.
When the deliverables are well defined including the purpose of the deliverable, the inputs required, and what the deliverable produces the project becomes self-evident.
The deliverables and their inputs define the project plan. The inputs could be from another task, from SME input, from external sources, from organizational artifacts. It is the inputs that allow the deliverables to be put together into a project.