I have been looking at rod blanks and some are described IM some number or other. 
Can anyone explain this to me? Jim
Jim,
I found this explanation that might help. Scroll down to see the info.
I do beleive that you could ask this same question on any forum which has info about rod building and get different answers on each one. As a matter of fact I have seen this question many times on fishing and rod building forums. So I will throw my 2 cents worth in anyway. There is more to a blank than the mfg modules/ im rating. Such as srim, taper, other elements added for whatever reason, design, mfg technique etc. In the past many high end rods have been made with what would be called âim6â. So I would suggest that if you have a particular blank you are considering for yourself, or are looking for particular performance, you might ask others on the forum what kind of results they have gotten. As you can see there are many good choices.
Like others have said, it gives some idea as to the graphite being used, but not much else. It doesnât address the resin, the rod taper, wall thickness, or anything like that.
IM6 is usually heavier than IM7 or IM8, but less brittle. You can get a general idea about the potential of the material, but it all comes down to how the blank was designed vs what you want in a rod.
One of my favorites happens to be a 60 mil modulus blank, but designed to be a moderate action with a fast recovery rate. To compare, I have a 57 mil blank that is lighter in weight, faster in action, with about the same recovery rate (my estimate) and not nearly as nice to cast as the moderate blank. Aside from my one 60 mil blank, most of my favorite blanks are IM7 or so. It all is about the blank and how it is designed as a package.
Hmm sounds like your favorite is a Signature V. If so I would say it is of my favorites also.
Yep. One of two âbig moneyâ blanks Iâve splurged on. Worth every penny. 
I thought each successive IM number was the ânext generationâ in graphite. I could be wrong, but was IM8 available 8 or 10 years ago when most were IM6?
Update:
Upon further research, check this out. It appears the IM number is the designation for the type of fiber used and it also appears that IM may be a trademarked name by the Hexcel Corporation http://www.hexcel.com/ (manufacture of said IM fibers)
Interesting specs from Hexcel.
http://www.hexcel.com/NR/rdonlyres/225E3CA5-965B-4B29-9A92-0CE184B1DDF3/0/CarbonFiber_SelectorGuide.pdf
BassBugâhow right you are!!! As you see,Hexcel will list their IMâs but as you go down the list, IM9 is only 42 mill modulus as the IM8 is 44, How can that be??? You have to take in the elongation, to failure or stretch too.
Iâll make it shortâCompanies will use terminalogies that most donât understand to sell rods. Donât get confused with the modulus ratings, IMâs or any other term being used to buy a rod.
Randy