soft, medium and hard refer to how early the tubes break up (soft yielding the most headroom, hard yielding the least)

Technically they will all work for light overdrive depending on how high you're setting the volume, but if you're turning the amp to 10, you'll be getting a lot more breakup from hard sets than soft sets.
^Truth, it also depends on what your bias is set at. I don't know how it would work for cathode biased amps though....
I don't give a shit if you listen to me or not
i think im gonna get a hard set of el84s becuase my main tone is like the edge from u2. it's cleanish with a slight breakup.