Actually it does mean something. There is a material difference between IM, HM and UHM CF. Typically as you start to exceed the 50 Msi barrier, you need to use PITCH type CF instead of PAN. These two types of CF are chemically and physically different as is the processes used to make them. PITCH type CF has high stiffness but because its crystal size is larger, it has a lower tensile strength. PAN type CF has very small crystals making it more flexible and giving it less modulus but it can be used to attain higher tensile strength.