REAL is very much a LEE proprietary design and stands for "Rifling Engraved At Loading".
The idea is ridges to catch the rifling down the side of the bullet, but with the ridges increasing in diameter toward the FRONT of the bullet: the basemost ridge will just barely catch the rifling, the next one forward moreso and so forth. The ridges are tapered in order to make loading the bullet MUCH easier than it would be with conventional bullets, which either would have to be patched or hammered all the way down. With a REAL bullet, you have something which LOADS almost as fast as a Minie but with a much greater potential for ACCURACY. The ridges are also designed to clean the bore while the bullet is being rammed home; some shooters report cleaning their rifle just once after a day's shooting, rather than cleaning several times during the day, once after each string.
It is because of their performance in modern Muzzleloaders that I was querying their use in BREECHloaders, knowing all too well the problems which the Royal Laboratory had with trying to make the Snider shoot as well as its muzzle-loading parent.