...
This series of images is ment to show that with a small enough cell size, each particle can be very well defined (in these renders the cell size was reduced to 2mm (a setting of 0.002), and secondly, how the Density Multiplier parameter can act as a sort of falloff between areas of density and no density. The Max Final Density was 500 for all renders. You can see that as the Density Multiplier decreases, there is a larger falloff between no density to the areas of maximum density. When the Density Multiplier is very high, there is a very sharp falloff, making the volumetric shape have much sharper contours.
Extra particles
This parameter adds extra particles around the simulated particles. The simulated particles must have some velocity as the extra particles will be scattered based on the trajectory of the simulated particles. So if your particles aren't moving, this parameter will not change anything.
Extra particles dispersion
The distance that the extra particles should be allowed to disperse from the original position of their "parent" particle.
Extra particles deformation
This has the effect of exaggerating the motion blur shown on the extra particles, making them move faster than their "parent" particle.
Motion blur factor
You can control the amount of motion blur visible on the particles, separately from the scenes main motion blur settings. If you want the particles to look blurrier as they move, increase this parameter.