Whereas Multigrade IV FB and RC had characteristic curves designed to cope with "hot" negatives (T-grain and core-shell "designer grain") that have rising curves, the new RC papers and their fiber based counterpart Multigrade Classic are much closer to "straight line." They work well with conventional grain negatives developed normally and designer grain negatives developed in ways that tame their rising curves, e.g. pyro, semi-stand, etc.
The new papers seem to be targeted to photographers who either use conventional grain films conventionally developed or designer grain films paired with tightly controlled darkroom processes. They're not as "forgiving" as their predecessors were. I suspect this was intentional on HARMAN's part as it evaluated how its customer base was changing.