The "X" scale does refer to line diameter. 0X is .011" diameter, so 1X is .010", 2X is .009" and so on. As far as which diameter I go with is about the same as when I used mono. For most of my steelheading, it is 2X stuff. If you go to the lighter stuff, let's say 4X or so, then it can't hurt (except in the wallet) to go with a stronger lb test to diameter flouro like Grand Max.