Positioning you wrote and tested
Product marketing hiring managers look first for proof that you can find the one thing a product should be known for, for one audience, and get others to use it. A letter that says “I created messaging” says little. Describe a positioning or messaging framework you wrote: the audience, the alternative they were comparing you with, the claim you chose, and how you checked it (win-loss interviews, sales calls, message testing, page performance). If the positioning later changed because the evidence said so, that is a strength to mention.