The Birth operator is the simplest operator for creating new particles over time.
Count: controls the number of particles to birth per interval.
Frames: controls the number of active birth frames per interval, within the overall time range.
Frames: controls the amount of time to wait between birth intervals.
The repeater is useful for repetitive particle birth sequences. For example, imagine you wanted to shoot 50 particles into the scene, every 20 frames. You would enable the repeater and set count to 50, duration to 1 and interval to 20. Using the repeater avoids having to setup multiple birth operators to achieve the same outcome.