I don't know all of this, but the im# (I believe) refers to the thinness of the graphite layer. A rod is most sensitive if the graphite is really thin but still strong. I think most high-end rods like G Loomis are IM10. That said, Just because a rod is IM10, it might be poorly made causing less sensitivity or brittleness (breakable). My recommendation is to go to the store, and ask the guys there whether the rods are returned often. Ask about the rods' warranties, especially if you are prone to breaking rods. Then, hold rods by their handles and drag their tips across cracks in the floor at the shop. You should be able to feel differences in the sensitivity. A rod being well made means more than what ever that call their materials.