I'm not sure to be honest, I spent my whole career with the kids who jump out of planes so I can't tell what the weird side of the Army does.
The Rapid Ready Brigades sounds like the Desert Storm era version of the Immediate Reaction Force (IRF), which is tiered 1,2 and 3. One being the fastest and three the slowest. The 12-18 hours is the Airborne standard and the 82nd will not let you or anyone else forget it. No ceremony in the 82nd is complete without some crusty SGM giving a speech about how no other Army unit can meet the 82nd standard and how you must carry your legacy with pride blah blah blah you've probably heard something familiar.
The idea for IRF being
IRF 1 your "spear head units" or more accurately your "Oh shit everything's on fire! we need boots on ground now now now" units
IRF 2 your heavy units who take time to deploy simply because you can only load tanks and shit on planes so fast.
IRF 3 your REMFs
Like I said I don't know the deployment standards for these categories and there all different but every Army unit falls into one of these categories and IRF 1 rotates between BCTs.