It sounds like they've described exactly what they're looking for in their letter to you - a specific type of 'cover letter' sort of thing that advertises the impact you will have on their business.
It's a rather unusual way to ask for a cover letter, but it also gives you an idea of what kind of cover letter they're looking for - one that cites experience relevant to the job, and describes how your experience and abilities are useful to them as a business.
If that explains what they're looking for to you, then write that cover letter highlighting your personal skills and the way they relate to the job you've applied for, then send it to her, attached to an email that explains that what is attached is your 'letter of impact'.
If you need more clarity than that, you could always call the project manager and ask them. Say something like:
Hello, I'm (your name), I recently got a letter from you after applying for a job, asking for a Letter of Impact. This is the first time I've heard of a Letter of Impact, could you please explain what you're looking for?
Try to be polite - this is something the manager has probably asked for from other people, and they might not have thought about the odd phrasing until now. Also try to have another question or two to ask, so that it doesn't seem like you called for something trivial - it might be more common in her line of work than either you or I are aware of, so don't be offended if she acts like it should be obvious. Just accept the explanation she gives and write the best letter you can.