The first digit is half of the fan width, the "5" in "517" equals a 10 inch spray fan width (5 x 2 = 10).
The last two digits are the size of the tip opening in thousandths of an inch, the "17" in "517" means the tip hole 
size is .017 inch
The RAC X range of tips are used for a wide selection of pumps and have a working pressure of 4050psi.