The operator is the basic element of the particle system: you combine operators into events to specify the particles' characteristics over a given period of time. Operators let you describe particle speed and direction, shape, appearance, and more.
The operators reside in two groups in the Particle View depot, and within each group appear in alphabetical order. Each operator's icon has a blue background, except for the Birth operators, which have a green background. The first group contains operators that directly affect particle behavior, such as transformation.
The second group, found at the end of the depot listing, contains operators that serve more of a utility function: Caching operators for optimizing particle-system playback; Display operators for determining how particles appear in the viewports; Notes, for adding comments; and Render, for specifying render-time characteristics.
The Particle Flow operators in the Particle View depot