To clear it up, the first number is the width that you want, the second number is the thickness that you want. You can't always get what you want, but it should be close (that is where the discreet thicknesses of the building blocks comes in).

Lets say that you wanted a width/thickness of 5,0.2, well there is no constraint on width, but there is a 0.4 minimum thickness for the building blocks, so you would have to decide if you would span the 5 width (this could be cm,mm...) with 0 thickness (rounded down) or 0.4 thickness(rounded up).

The pair 3,0 is different because it has 0 thickness.
So if you were builing a 2 humped shape and had a 0 thickness in the middle, you could then look at it as 2 seperate shapes, but if it was a small thickness, say 0.4 in the middle, then it would be 1 complete shape.

I hope that this clears things up.