Velocity Curves - What aspect of timbre is actually changing?

With respect to the vertical axis in velocity curves, what aspects of timbre are actually changing? 

Are they included in the note editor now? 

If not, that would be very powerful to allow these characteristics to be varied across the register of the piano, as they greatly control softness versus brightness - these would be ideal to change at varied points from bass to treble of many pianos.

- David