Let's say that a one weight works best when using 8X tippet, then a two weight would use 7X, three weight 6X, four weight 5X, five weight 4X, six weight works the rod best when using 3X, and so on. So by matching the rod to the tippet, using this method, lets us use a tippet that best matches the flex of the rod when fighting a fish.
Smarter words havent been read by this angler yet.. thanks Fox!
best fishes - Brian