Hello and welcome,
Like others have said, the review process changed in October 2023. Here’s the news post from when it changed, if you’re interested:
If you compare the new system to the old one, Yes would be similar to a 4-5 star rating, I Don’t Know would be similar to 2-3 stars, and No would be similar to 1 star. I find having 3 choices a little easier to understand than 5. I also like the breakdown of answering 7 questions, all with tooltips to help us understand what it’s asking if we have any questions.
And yes, if you are unsure or there is a grey area while reviewing a nomination, don’t be afraid to use I Don’t Know. Always use your best judgment while reviewing, based on the criteria.