The terms of reference (ToR) is a key document to support the governance arrangements of the project. The ToR for the project board would usually be defined at project brief stage.
The terms of reference follows good practice in project management and is the document that sets out the project roles, responsibilities and membership names of the project board. It can also define the roles of the delivery level project team and any associated programme or executive level governance that the project board will report to.
The project manager produces the terms of reference in conjunction with the project assurance (if applicable) and the appointed project, programme or executive board (if applicable).