Upfront, I just want to call out something that you all likely already know - PMM’s strategic and interconnected role makes it difficult to pinpoint and measure impact. Strategic, foundational work is hard to measure. And being so interconnected means that there are heaps of dependencies and variables. Shared goals and metrics are part of the job, so get comfortable with that. Build communicative, productive partnerships with the teams that have shared goals and align on the approach. Celebrate successes and failures together, do post-mortems, and learn and improve hand-in-hand.
Now, onto the question.
PMM should be responsible for all activities related to customer and market knowledge (market research, competitive research and differentiation, target audience definition, etc), messaging & positioning, and teeing up the launch by providing briefs and foundational materials that enable channels (web and marketing assets, sales readiness and enablement, PR, demand gen, etc).
Demand Gen should be responsible for all activities related to the performance of the campaign (SEO optimization, SEM, promotions, direct advertising strategy, email marketing, and paid ads).
Because demand gen will rely on product marketing to support campaigns by defining what products/features/experiences to highlight, for which audiences, and what language to use, the work is inevitably linked. It is clear why KPIs like CAC (customer acquisition cost), LTV (lifetime value), win rate, marketing/sales cycle length, campaign metrics, etc. are often used by both demand gen and product marketing -- they span the work of these two teams and many more.
Imagine you have a goal of making a delicious cake. To do so, you need high quality ingredients (PMM) and a skilled baker (demand gen). If the cake doesn’t come out well, it will be impossible for the person eating the cake to understand what went wrong -- but, you can input tests along the way to get signal. Test messaging using surveys or a customer advisory board. Run small experiments before investing a lot into demand gen to understand messaging’s impact on conversion. This can be an indicator of whether you’ve got quality ingredients.