The X rating refers to the diameter. If you subtract the X number from 11, that will tell you the diameter in thousanths of an inch. For example, 3x tippet material would have a diameter of .008".

The origin of the X number dates back to when leaders were made of gut. A piece of gut would be drawn through a hole which would set the diameter. If a smaller diameter was needed, the same piece of gut would be drawn again through a smaller hole.

So 1X leader was that which was drawn through once, 2X leader was that which was drawn through twice, etc. That is why the diameter is smaller for larger X numbers.