Functionally there is no difference.
The situation arose from the old ?25% new work,? ?40% old work? rules that were in place over 30 years ago. The maximum raceway fill for ?new? work was limited to 25%. When a raceway was reused, up to 40% was permitted. Then it dawned on the Panels that if 40% were safe at all, there was no valid reason to limit new work either.
It took UL over 25 years to realize that their test standard for seals was based only on the 25% fill and word slowly leaked out. The UL test standard is predominantly a performance spec and many of the old ?25% seals? are still very close performance-wise to what are now ?40% seals,? so the difference may not always be discernible. Their sizes and volumes were larger for reasons that had little to do with the fill and more to do with the casting/manufacturing process. One of my IEEE/PCIC papers with Lon Ballard, the Crouse-Hinds, Manager of Technical Specifications, explains this in detail.
Several of my Proposals through API lead to the current text including, recognizing 40% designs, recognizing ?ring? reducers in oversized seals, and recognizing it is based on RMC fills specifically.
