Each level has a higher modulus that is more sensitive and a faster tip(flex).
GL2 IM6
GL3 IM7
IMX IM8
GLX IM10
GLX's a lot of them exploded when they first came out. Don't know if they have gotten that fixed or not. I have a few IMX's
Question to go a little deeper into this topic? A IM6 & 7 are pretty cheap blanks, why are they so much $$$ in the Loomis series? It seems like you can buy a very good rod in IM 8 & 10 and still be cheaper than the Loomis rods, is a person paying for the name or are they really better. A example is the Shimano Cumara is a IM 10 and it's $225.00 but the same rod in Loomis is over $350 (if I remember correctly).