[plt-scheme] [ADT] : Modules or Units ?

When implementing an abstract data type with DrScheme, one could
choose a module, a unit or a class.

A class could be a class.ss-like or a closure with local state.

On the non-object side, I would have chosen a module, but a recent
post in this list used a unit for the ADT Queue.

Can somebody from PLT elaborate on the pragmatic distinction
between units and modules ? And their links with teachpacks.

Thanks a lot,
      JP. Roy