> Why it's not possible to draw a Circle "tangent" (with function circle tangent)
> between 2 rectangles aligned (or not) or inside one rectangle?

Hi Pilou, this seems to work over here:

Are you possibly clicking on the same line for both picks? The way circle tangent works is it needs to have 2 different tangents, it won't work when 2 picks are both on the same line segment - there are an infinite number of circles tangent with the given radius to the same line. However, you can use Circle/Diam to draw a tangent circle to a line in this case.

> a side separated it's not a curve?

Yes, a side of a rectangle that has been separated is a line curve object just as if you had drawn it with the line command.

- Michael