The bounds are used for culling – when the bounds overlap the view frustum, the particle system is rendered on screen.
Some particle systems also use whether they’re rendered, to determine whether they are simulated.
Fixed bounds let you get more control over this than you’d get from the automatic determination, either for performance (dont’ simulate/render a dust system just because a single particle happens to have flown very far out,) or for including things that automation might exclude (especially if particles are expanded in the shader.)
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