Question:
What's the difference between breaklines and subzones?

Answer:
Subzones are for specifying different parameters to a subarea (eg max down slope) where as Breaklines can be used to modify the design surface after it has been calculated.  Eg you can define a channel or levee in to the design surface.  Along with the ability to delete out points from the OptiSurface grid, breakline allow you to modify the DTM to any surface what you want.