I believe storm says its NRG without the additives/particle. I wonder this myself, r2s is r2x without additives, erg is NRG without the additives. R3s is R3x without the additives. I assume its 3 different covers, I think ERG is reverse engineering the NRG cover tweaking it to make it THS friendly.
I have the crux being the strongest iteration of the ERG...the rest of the erg's I have owned...lucid, zero, byte were all fairly weak IMO.