Thank you very much for your reply!!!

But I'm not sure if it is also useful if the shops are not moving in the surface accessibility,..

The boxes are never moving in the Y direction, but they want to know the distance from them to the accessibitity surface in the Y direction.

Is it ok then? should I use

float $pos[] = `pointOnSurface -u $u -v $v`;

to know the position of the box in UV coordenates?

Thank you again,.. it is very useful for me,.. I'm very happy to get some answer :-)
(I need to solve this before friday,...if not my teachers will kill me, because I'm doing architecture, and they are not interested with that,.. they feel that I'm loosing my time, But I think artificial intelligent can be very useful for architecture!!!!)