According to Kirk (2016), The essence of “Formulating Your Brief” is to “identify the context in which your work will be undertaken and then define its aims: it is the who, what, where, when and how.” It could be formal or informal as any project you think you must make it. This phase is where you create a vision for your work.
Reference
Kirk, A. (2016). Data Visualisation: A Handbook for Data Driven Design. Thousand Oaks, CA: Sage Publications, Ltd.