Apparent Size Modifier modifies the size of particle according to its apparent size.
The main purpose of this modifier is to reduce noise for very small particles. When visualizing simulations with large number of particles, apparent size of each particle can be very small, i.e. smaller than 1 pixel.
In such a case, the image of video can be very noisy one, since whether particle appear in a chosen pixel depends on some probability.
With this modifer, you can fatten size of particles with small apparent size, and as a conpensations of fattening, you can modify its alpha value of the particle.
Also, you can set the maximum limit to the apparent size. You can keep star particles as small dot even if an star is accidentally distributed on ahead of camera view.
Comparision between noisy particle data with and without modification.
*1 I mentioned above that this modifier use apparent size, however, apparent size of particles depends on many factors, such as resolution of canvas or rendering type.
Thus. we use size divided by distance to the camera as a internal parameter, instead of true apparent size.
Thus, note that proper values for parameter can be different for diffient canvas resolution or rendering type.
*2 Since this modifier use distances between particles and camera, the distance must be calculated in rendering process.
Since distance information is calculated in sorting process, you can use this modifier only in rendering process with sort.