Each line, is used for different core characteristics.....
The Premier is Asymmetrical, with the RAD, and now the Shape Locks.
The Master is Similarly numbered (RG and Diff) but used symmetrical cores. These provide (typically) earlier roll then the Asyms, and are moderately priced less. Usually because you pay more for more R&D wich is required when designing an Asym.
Think of these as a 1a and 1b for high end lines.
The Fire Line, Used the Twin V cores, provide more length (higher RG) and some less flare (diff) GENERALLY providing more length out of the ball, assuming things like Cover make up / prep , layout, release, oil yadda yadda are all the same.
Thunder, is VERY similar to the Fire, core number wise, just providing a little more POP on the back end,
Hot, is the next step down, core wise, even smoother then the Fire line, this one will typically use a slightly weaker cover stock just to create more of a gap between the two lines above it.
I think you can see where the Tropical is going.... less flare more built in length, and again using a weak-ish cover
This is just a generalization however, as the core numbers really only determine the SHAPE of the reaction and not HOW MUCH reaction, thatch done cover wise.
Storm only ever has 2 / 3 covers in use, usually cross all or most lines.
and as they develop stronger coverstocks, the older ones get moved down the line.
As you see R2S in the Hot line, when it was previously only Reactor,
R2S is being replaced by R2X in the mid lines, (likely as new info the the reigns has not yet been released)
and probably a new Premier debuting mid season with a new stronger cover...
My own opinion here: there is a flaw in this system as the old covers are by no means WEAK, and therefor low end pieces become typically too strong for truly burnt lanes. You will hear alot of war stories of people Tropicals Being "HOOK MONSTERS" because the the covers are just plain too strong...
But I could be way off with this and I am sure I will be corrected, I have owned @least one ball from each line, and have sold many others. and heard the same things. Just what I see...
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In The Bag:
http://www.putfile.com/rabbit69ca Storm: Dimension,Paradigm, T-Road Pearl, Ice Storm
Lane 1 Liberator
Columbia 300: Arch Rival, Scout/R Hi-flare
Ebonite: Playmaker