Particles

Small points of light hang in the air and drift through the frame. The air stops being empty and becomes a thing the light travels through.

It puts depth between the camera and the subject. Particles at several distances give the eye a series of planes to read, which is why a plain room feels deeper the moment they appear.

Backlight them, as the note says. A particle is a tiny object and it is only visible when light comes through or across it, so a source behind or to the side is what makes it appear. Front light leaves it invisible against the background.

They drift rather than fall. Real particles this small are carried by air currents, so the motion is slow, slightly upward in places, and never uniform.

Depth of field does most of the work. Particles near the lens are large soft circles and particles further back are sharp points, and having both is what stops the effect reading as a flat overlay.

Use it for a shaft of light through a window, for a workshop, a forest, a stage, and for any interior that needs air in it.

Skip it in a clean, controlled environment. Particles say the space is old, dusty or smoky, which is wrong for a laboratory or a new car.

Volumetric light is the beam itself, a visible cone or shaft of light in the air. Particles are the objects that make a beam visible. The two arrive together, and they are not the same thing to ask for.

The test is what you can point at. A shaft with edges is volumetric light. Individual specks you can follow are particles.

The particles come back invisible because the light is in front of them. Say the source is behind or across them and that they catch it.

The second failure is a flat sheet of identical dots. Ask for particles at different distances, with the near ones out of focus and the far ones sharp.