It is possible to assign particles to different groups.
This is useful because you can use a Question object to test whether a particle is in (or not in) a group and trigger an action for that group.
You can also determine which particles are affected by a particular modifier.
Groups tab menu.
Set as Random, by default.
There are three other options: Sequential, Use Gradient and First Group Only.
This animation shows an emitter generating particles, which are being randomly assigned to one of two particle groups, with the Mode set at Random. When xpTurbulence is enabled, it is applying turbulence only to Particle Group 1, as per the Groups Affected tab instruction.
When particles are created, they are assigned randomly to one of the groups in the Groups to Use list.
The first particle is assigned to the first group in the list, the second particle to the second group, and so on.
When the end of the list is reached, it wraps around to the beginning of the list again.