Points can be disks, spheres, or quads.
A 3d vector array of the positions of points.
A float array of radii.
A float array of aspect ratios.
A float array of rotation angles (in degree).
Choose to render points as disks, spheres, or quads.
If this value is non-zero, particles in points mode with a small on-screen width will be automatically enlarged so that they are at least the specified size in pixels. The enlargement fraction is then used in the particle shader to adjust the opacity so that the visible thickness of the particle remains the same. For a given number of AA samples, this makes it a lot easier to anti-alias very small particles, at the expense of render time (because of the additional transparency/depth complexity). Good values are in the range of 0.2 to 0.7. Values closer to 0 are faster to render but need more AA samples. So if your scene already uses very high AA settings, you should use a low value like 0.1.
Particles switch to rendering in volume mode when using this attribute. This controls the rate at which the volume shader gets called. The value indicates the average distance between adjacent volume shader samples along a given ray, in world space units. Generally, you will want this setting to be as large as possible before visibly noticeable artifacts occur, and in the case of voxelized volumes no smaller than about half the size of your voxels (in world space). A low step_size will increase render times. For example, when the step_size is 0.1 and the volume is 10 units big in world space, there are approximately 100 primary samples and therefore the volume shader is called 100 times.