Where in the emitter's local space a particle is born. Without one every particle starts at
the emitter's origin, which is right for a hit spark and wrong for anything with an extent:
the width of a forge, the mouth of a flame column, rain across a room.
A rect spreads births uniformly across width x height; an ellipse spreads them
uniformly inside it (the square-root of the radius, so they do not bunch at the centre).
Both are centred on the emitter and move with it, since this is local space.
Where in the emitter's local space a particle is born. Without one every particle starts at the emitter's origin, which is right for a hit spark and wrong for anything with an extent: the width of a forge, the mouth of a flame column, rain across a room.
A
rectspreads births uniformly acrosswidthxheight; anellipsespreads them uniformly inside it (the square-root of the radius, so they do not bunch at the centre). Both are centred on the emitter and move with it, since this is local space.