All of the tasks listed on the project plan need to be assigned to an individual or team. The person or persons responsible for a task will need to know in detail what the task involves and also the dependencies and timescales, which can be clearly communicated to them using the Gantt Chart.
The business requirements document should accurately, and in detail, describe the purpose of the project. It states what is needed to achieve that goal i.e. what is in-scope, what is out-of-scope, any assumptions that have been made, any constraints that have been imposed and expected timescales.