The settings of the particle-system beside the preview-area will be adjusted by the flag "system". They influence all particle-emitters.
Scene for preview
You can choose a scenery for preview which is positioned in the back. It serves as an orientation for the particle aspect in the foreground, but cannot be addepted. The adjustement for the particles is to be executed in the foreground scenery. So you can use a particle system in several sceneries (but always at the same position, for example snow and rain).
Particles are "real" 3D-objects, that means they move in an area defined by three coordinate-axis (x,y,z) and are observed by a virtuell camera. At the beginning a particle-emitter is located at the point of origin and the camera is located 3 units away from this point on the z.-axis. The camera is geared to the direction of the positive Z-axis, to observe the point of origin at 3 units range, -3.0 has to be adjusted (presetting).
Before it is shown for the first time, the particle-system has to be fast-forwarded by the adjusted lead-time. Otherwise (for example) rain would restart to fall downwards the screen-border at every re-enter.
Every emitter can be affected by a force field. The activation of the option shows us the exactly position and direction of the force inside the preview area.