There is a long-case line, as well as a shortened version designed to fit a short rifle action (such as a .308 Winchester).
The shorter cartridges are known as Remington Short Action Ultra Magnum, RSAUM, RSUM, or SAUM.
The RUM family is based on work done creating short magnums by John Lazzeroni, similar to the Winchester Short Magnum line.
Repeating Arms Company was the first major manufacturer to commercialize Lazzeroni's concept, followed shortly thereafter by Remington with their "Short Action Ultra Mag" line.
Another influence was a series of 6 mm cartridges developed for bench-rest target shooting competitions in the 1970s.