A project begins when the sponsor/business gives the project manager (PM) a mandate to develop the project brief. If this is a major project, a mandate should have already been completed and approved through the integrated planning process.
The project manager will meet regularly with the sponsor to ensure that the project develops in a way that the sponsor supports. The PM then writes an initial outline of the project background, objectives, scope and exclusions in the form of a project brief. This is approved by the sponsor to check it accurately reflects what they've been asked to do.
The project manager begins initial stakeholder identification and talks to user group representatives to help them understand the aims of the project and develop the brief further. They begin to elicit details of what customers and users expect of the product or service being developed.
Requirements gathered now are about what customers expect the product to do or to be like, not technical details or technical approaches to how the requirements are achieved. Those will come later in discussion with technical experts.
If there are a lot of stakeholders the project manager may seek their sponsor’s approval for early engagement of a senior user to help with requirements gathering. The PM and senior user will work with the users to prioritise long requirements lists into essentials and desirables. This forms the basis for developing the acceptance criteria - those qualities that the deliverables must have in order to be deemed fit for purpose and, without which, it will be rejected.