[plt-scheme] When would you use units rather than classes?

On 04/04/2008, Grant Rettke <grettke at acm.org> wrote:
> I was reading the unit documentation in v372.
>
>  MzScheme's units are used to organize a program into separately
>  compilable and reusable components."
>
>  My first thought to accomplish such a thing would be to use classes.
>  Clearly, units are not classes, so there is a different intent.
>
>  What is the intent? When would you use units rather than classes?

I am curious as well, though I'd like to note that modules belong in
this discussion as well, as "seperately compilable and reusable
components."

    Henk