Who does product and project requirements and why?

Take a look to this post: https://www.projectmanagement.com/blog-pos...olio-Management
You have to talk about accountability because the responsible is "the Doer" using the RACI jargon. Project manager focus is work needed to create the solution. Business Analyst focus is the solution. The relation between both roles are because most of the organizations define SOLUTION = "the thing" (product/service/result) to be created PLUS "the work needed" to create it (project). Remember that "product" is using as "something to be produced".

Great!

Please let me say that BA does not own the requirements. Here is the critical component to understand. Project requirements owner is the Project Manager. Solution requirements (mainly product/service/result requirements) owner is the Business Analyst. BA must give to the project manager all needed to define, plan, control, close the project. The project manager will define assumptions and restrictions from the information the business analyst provide too.

Let's be practical here. A PM is selected to manage an identified project. Failing to do at least an initial business analysis to determine if any requirement exists is not logical. That would say the PM creates his/her own requirement, forms a team, then solves their own defined requirements.

In the practical world, that is not how requirements are generated, through analysis of a need for change, followed by a decision well above any PMs level to pursue that change, and the PM inherits the initial BA for use in further fleshing out their approach to solving the requirements. That may include additional analysis by the BA assigned to the team.

Bismarck:
Actually, it's really in the hands of your C-Suite until a core team is assigned but many here all agree on the normal standard practices.
Can you clarify the following:
Are you the client/customer or the system integrator/vendor? Is your question about the client or the vendor?
What do mean by put in charge and what phase of the project are your referring to?
Are you developing the business case, feasibility study or concept paper?
Are you developing the project charter with the sponsor?
Are you in the process of developing the RFP, RFQ or RFI and put it out to bid?
Thanks.

Its not a matter of assumptions. There are standards and roles definitions outside there and I am working helping organizations to implement both roles fron 1990 up to date. So, all you need is outside there. Including lot of implementations inside lot of organizations.

Agree John.

Organizations don't cater to standards; they embrace them, adopt them, refine them as their standard practice and embed them into their organizational culture. All PMs, BAs and sponsors adapt to their culture and continue to work to bring best practices into their organizations.

The project exists because a business analyst have performed the activity to define the solution, create a business case and get the approval for the project.