I've usually seen the sizes larger than 0X referred to as 01X, 02X, 03x, etc. So you might see this nomenclature instead of that presented by Juro above.
My understanding is that the "X" designation system is a carry over from the old days when tippet was made of gut and drawn through a die to creat the desired diameter. The" X" referred to the number of times the gut was drawn through a die. So, a 3X was drawn through three times, a 4X four times, etc. Makes sense if you think of the X like a multiplication symbol, e.g., 2 x 2 = 4 = 2 times 2.