In the middle of the Generator Param1 menu, a table lists the different particle generation properties.
(a) Particle generation properties
For example, if you set the number to 0.1, a particle is generated every 10 frames. The exact frame within those 10 frames when the particle is generated is randomly selected. If you set the number of particles to 0.3, a particle is randomly generated approximately every three frames.
Each parameter has a variance value used to specify how much the parameter varies each time particles are generated. Exactly what gets varied depends on the parameter.
For example, if you set the Number parameter to 10 and its Variance parameter to 5, the number of particles generated at each frame is randomly selected between 5 and 15.
The unit of measurement used by each Variance field matches the parameter it is varying. For example, if the Number parameter is expressed in particles per frame, so is its Variance field. The Speed parameter is in pixels per frame and so is its Variance field.