Not sure I understand it either Wilfy, my understanding was that if something was flat say like a circle drawn on a piece of paper it was 2D because it had length & height but no depth, 3D was something that had both length, height & depth so the same circle cut from a solid block of material was 3D.
2.5D was something that had width & height & the illusion of depth as used in computer games when flat objects appeared to have depth.
Problem with that is people talk about 2D machining but I can't understand how it can be 2D when you are creating a 3D shape lol