Fixing incorrect requirements
You have to get feedback on any requirements you weren’t sure of. For any load limits you plan to measure and any failure modes that could be handled differently, you have to ask for confirmation explicitly. As a tester, you need an answer to those questions, but you might not be best placed to make those decisions. The user experience team and product owner will have a better idea, so ask them the specific questions you need.
As previously noted, reviewers may object to choosing a limit for system metrics. They may say there’s no point stating that the system can only support 1 million users, for instance, when in all likelihood it could do many times that number successfully, or conversely, it might not get anywhere near that number in reality. While there may be no difference between 1 million and 10 million users for the development team, there is a large difference in what you have to test and support, so you need to agree on that...