Thank you for your help, Rob.
I've followed your suggestion and found that it is true Ansys Fluent will receive data (in UDMs format) from Rocky during the simulation.
According to Ansys Rocky User Manual (21.2.0), an "Additional Inputs" lists items that Fluent will receive from Rocky during the simulation, which Rocky instructs Fluent to create as User Defined Memories (UDMs). And "Additional Outputs" lists the items that Fluent will share during the simulation with Rocky.
However, on my Variables tab, nothing showed.
If I understand correctly, I must create an additional input on the variables tab.
Could you please guide me how can I define the variable I need (in this cas is VoF of particles) in the variables tab to pass back into Ansys Fluent?