Thank you so much for your help, Rob.

I did not use CFF. I used an available MPM techniques in Ansys Fluent 2022R1.

Below pictures are the way I created a group of particles.

After that, I initialized a system.

Then I found the problem that I already explained above, a contour of a MPM-volume-fraction looked logical but a contour of air volume fraction did not make sense.

I am still curious that

(1) Do I understand correctly that Fluent already recognised there is a group of particles in a domain after I initialized MPM functions and particle (fig 6 above).

(2) If the MPM volume fraction was already recognized by the software, why the air volume fraction is full on plane 3?

By the way, I will check at the Theory Guide as you suggested. Thank you again for your valuable time.