I haven't done that particular job title but I have been a Product Planning Manager (at Sony for TVs). My impression is that it is a very hard role to describe

I haven't done that particular job title but I have been a Product Planning Manager (at Sony for TVs). My impression is that it is a very hard role to describe because it includes "whatever needs doing" and will vary by product and the team around you. It will include making sure everything gets done and tied together including not just the product itself but the documentation and any licensing agreements and ensuring correct marketing information is prepared for the product at the right time (and the marketing team understand the product). It may include what is normally considered business development and associated liason with legal. It may include booking and preparing for a trade show. In a big company playing a linking role and getting the right people talking can be important even if you then step back. Very importantly it should be someone able and prepared to take decisions that might not be big and require senior management but that need making and aren't really either engineering decisions or design decisions.