Hi Pilou,

> That is very mysterious now that works even I put the copy on the right,
> on the left, in front of, or even behind!!

It's only in "Projective" mode of surface-to-surface Flow that the specific placement of the base or target surface makes any difference.

In the regular mode that's being used here, it does not make any difference specifically where you have located the target plane.

If you're seeing differences it's probably because you're clicking in slightly different spots on the base and target surfaces - the pick point for where you click each one is significant, see here for an explanation of where to click on each one to match things up properly:
http://moi3d.com/forum/messages.php?webtag=MOI&msg=5853.1

- Michael