When a prototype costs a day, the expensive decision moves to before the build: which problem, for whom, against what alternative, and why they'd switch. That's positioning, and it's also the first half of the problem statement. So they're one file now.
The PM owns the problem and the evidence. The PMM owns the alternatives and the claim. The file lives in the repo next to the spec, where the agents read it. Emily Kramer's analysis of more than 1,100 job posts shows the PMM seat splitting into systems builders and campaign runners, and the systems builder is the one who belongs with the build. Both owners answer to the CPO.
The full argument is part 2 of The Product Marketing Transition, which follows part 1 on why product marketing reports to the CPO. Every part is on the series hub, and the CPO side of the case is in The CPO Transition.