The X-rating system is used to classify the diameter (thickness) of tippet material in fly fishing. The system ranges from 0X to 8X, with 0X being the thickest and strongest and 8X being the thinnest and weakest. The X-rating helps anglers choose the appropriate tippet size for their target species, fly size, and fishing conditions.
Not all manufacturers use this rating, some indicate the diameter of the line in inches and millimeters. The manufacturers that use the X-rating also use the inch/mm indication, so you can always easily compare tippet and leader thickness.