I was under the impression that the 3D aspect of 3D (versus 2.5D) was the contour profile/zig-zag operation because since it is generated from a 3D shape (i.e. remove all the material down to the contours of this shape). Of course, now that I think about it, a difference between normal pocket and profile operations and the contour profile-type of operation is that the in the former the spindle is only moving in the X and Y axes at any given time, with specific Z axis changes during step downs (perhaps only worth only .5 D?); while the latter is potentially moving in all axes at the same time.
But when I say "under the impression", I mean that my knowledge of the subject is largely woven out of supposition, and based on facts that may or may not have been pulled out of my nether regions.
Talk about all things CNC