From what I’ve seen U-axis is barely even mentioned in Ruida manual. I only saw this referenced in an old forum post.
I believe both 6445G and S can use U-axis for rotary. I saw something where LightObject commissioned or worked with Ruida to get that capability on the 6445G:
Update patch for Ruida R6445G. Enable a rotary to the 4th axis (U axis) : lasercutting (reddit.com)
I wish I’d known about the limitation of the 6442G before purchasing. I only saw that it was 4-axis. Didn’t realize U was reserved. I also don’t see any reason why Ruida couldn’t update the firmware to support this just as they did with the 6445G/S.