This page provides some details on the settings available for the Point Particle Material in V-Ray.
Diffuse Color – Specifies the color of the particles.
Use Particle Color – When enabled, the material attempts to take the color from the native particle color channel in the Alembic file, if such a channel is present.
Phase Function – Specifies how light scatters from the point particles. A value of 0.0 means perfect diffuse scattering in all directions. Values close to 1.0 specify that light scatters predominantly in the direction that it comes from. A value close to -1.0 specifies that light scatters mostly backward. The phase function is useful to simulate scattering from water mist or steam, in which case values between 0.0 and 1.0 work best. For dust particles, smoke or soot, a value of 0.0 is more appropriate. The particular phase function model that is implemented is the Henyey-Greenstein model.
In the images below, we have particles randomly distributed in a plane, with two spherical lights behind and in front of the plane. The images show how the particles scatter the light coming from the spherical lights depending on the phase function parameter.