Any point or set of points can be used as the basis for the particles in a particle system. Grid SOPs or Sphere SOPs are often employed for this purpose. A particle system can be created in SOPs by using a Particle SOP, particles will be emitted from the points of the geometry connected to the Particle SOP's first input.
Any point or set of points can also be converted into particles using a Convert SOP.
See also Point Sprite MAT.