Your question is Test Planning With Sparse Documentation. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Tell me about a time you had to create a test plan for a highly integrated hardware-software system with little or outdated documentation. How did you decide what to test first, how did you close the knowledge gaps, and what was the outcome?