Thank you Jeremy for the response. It makes sense now. I tested it on a unit cube, with quiescent water and a specified rate of release of particles from one of the sides of the cube. I did a transient simulation for 10 seconds. After the simulation, I calculated the volume integral of Scalar-0 inside the cube and it is matching with the rate of release times the simulation time and the area.
The quantity that I am looking for is the number of particles that cross a given surface, say an interior boundary or a physical boundary. I couldn't find an option to do this. I tried surface integral of Scalar-0, but it gives a large number because Scalar-0 is the number of particle per unit volume. How do I calculate the actual number of particle crossing a surface?