Hello Michael,

Ok, thank you for all your answers. That fits gradually in my head.
Yes, I had unterstood that "Flow" ran like the mapping UV texture.

In fact, my perception of the NURBS was wrong, I saw NURBS like a sort of Illustrator in 3D; and the verb "to trim" bad translated by me, in context of NURBS, confused me.

So, all the curves are like tensed up, drawn or bend, or stretched in a cubic or parallelepipedic space. It's why a circle needs more place than a square.
To learn that a square is a tensed surface gives me a bizarre (odd, strange) apprehension of the concrete objets that we built in 3D; it's very virtual suddenly; almost incredible.

Yes, the new projective option correspond to that I tried to do with "Flow". This last novelty goes to simplify greatly the work of modelisation. Super!

Still thanks, have a good day.