The "Ultra" denotes the wider feed rails in the action. There are two widths available in the Long Action: Standard, which fits the 300 Win Mags, 7mm Mags, etc..., and the Ultra Mag width, which fits the RUMs and Noslers, etc...What is an "Ultra" under the long actions? Haven't seen that one before and am curious since I like the RUMs
If you try to feed a RUM or Nosler through a standard width rail, it will not be able to pass through from the mag box to the receiver, as it will be too fat. The Ultra feed rails are wider and will allow them to pass through.
If you are using 300WM with a detachable mag, or Wyatts center feed box, you can use the Ultra action, since the magazine will control the round instead of the feed lips.
Hope that makes sense! -Brian
Also, both standard and Ultra have the normal .550 bolt face for Magnum cartridges.