This is related to particles sorting, but instead of sorting individual particles, make it possible to be consistent with the rendering order of sets. So that user could affect:
1) If all particles (of system / emitter / particle) are rendered before or after some model.
2) If all particles (of emitter / particle) are rendered before or after another particles (of emitter / particle).
One example of 1) is EmitterShapes.qml. Increasing models opacity shows how particles are sometimes rendered before and sometimes after the model (video1).
One example of 2) is Fire.qml. After changing 0 -> 10 into emitters directionVariation z, sometimes smoke is over and sometimes under the fire (video2).
Try to solve this for 6.1.