The hardness property moves the brush falloff towards the edges, making
the brush sharper.
This should be the intended way to control the brush falloff instead of
tweaking the falloff curve manually, as it can be mapped to pressure to
make the falloff variable during the stroke.
It is also a good idea to show in the UI that the custom curves is an
advance features and it should almost never be modified when sculpting/
painting unless you want to create some advanced effects. By modifying
the curves freely it is really easy to break the brushes and make them
produce artifacts. This needs to be done in a later after merging the
pending projects to reorganize all the brush properties accordingly.
This was already implemented in the sculpt vertex colors patch, but it
is better to merge this now independetly as it is needed for clay strips
and some other brushes.