If you hover over the column heading, a pop up appears with a brief outline of the differences between the fares. There's also a link in the pop-up that brings up a window that explains everything.

Looks like voucher only.

I'm not an expert on all the refund / voucher rules in existence but what I will say is that this new interface is a huge improvement over the old one. Really nice seeing everything on one screen, especially for those that are usually traveling LD in sleepers.