It means that we can record MIDI performance data while recording the piano acoustically.
Hypothetical example:
After playback, we notice there’s some drum bleed in the piano. No Problem!
Once everyone leaves, we can playback the MIDI data into the piano and record the acoustic piano "after the fact" without bleed.
We can also correct any mistakes in the performance-
The midi data recorded during the session can also be used to trigger another piano sample from our extensive library to layer or perhaps replace the acoustic sound if so desired.
And if that weren’t enough,.. Here’s another great feature: Clients who perhaps don't have an acoustic piano, can send us MIDI data, we can send it to the Disklavier, and it will play your performance back as you played it. We can send you a recording of your performance using the acoustic piano.
So many options!