A suit index is a rating system for men's suits. It's a way to compare the different quality, fit, and style of men's suits. It works by assigning each suit a number between 0-10, with 0 being the worst and 10 being the highest quality. For example, a classic wool suit might be rated with an 8, while an off-the-rack suit might get a 5. The higher the number, the better the suit is. That way, you can quickly compare different suits and decide which one is best for you.