That sounds correct... each Part covers a different Note and/or Velocity Range of the instrument. Because as a human the Velocity you strike a key at is fairly arbitrary. Once you realize that it’s Velocity Range that is causing the sometimes it sounds / sometimes it doesn’t, it starts to make sense.

Here is what’s going on... the MODX lets the programmer build huge multi Part sounds... each Part can have as many 8 Elements (Oscillators) ...so if you want to build a sound with more detail, more nuance, this synth engine allows the programmer to go wild and build instrument sounds with additional Oscillators.

In this Performance, the programmer decided to create more nuance by using multiple PARTs of a Performance (4) to break the limit of 8 Waveform Elements for a sound. Because each PART can have as many as 8 Elements, each PART builds a different portion of the CFX Concert Acoustic Piano sound, allowing for more detail, more velocity switches. There are now 18 Elements in total: There is a 9-way velocity switch setup across the main body of the piano (up to G5), and an 8-way velocity switch setup on the notes above G#5 (those without dampers), plus there is a Key-Off Sound Element.

This version of the Yamaha CFX piano has much more nuance and detail in the soft, medium-soft, medium, and medium-loud velocity layer ranges, and is ideal for solo piano work where this detail can be heard and appreciated.

The “CFX Concert” occupies four PART slots. Each contains only a partial instrument. When performing “live” you are not as concerned with how many slots or MIDI channels this occupies, you simply want the best sound available!

Analysis:

PART 1 only has data across the body of the instrument up to the note “G5” and is a 6-way velocity switch through to a maximum velocity of “92”. It cannot be used alone – it only makes sense used with these other components.
PART 2 only has data across the keys up to G5 and is a 3-way velocity switch starting at higher velocities “93” through “127”. It cannot be used alone – it only makes sense as part of these other components.
PART 3 only has data above G#5 and is an 8-way velocity switch in the undampered area of the piano.
PART 4 only has the Key Off Sound – the dampers and felts falling back in place on notes up to G5.

Extra Credit:
Mastering MODX Performance Basics 1
Mastering MODX Performance Basics 2

Thank you for the fast and detailed response. I will study the details of your answer.
Thank you!