in manual, emission attribute : When in surface mode, this point attribute will be used to vary the chance of any polygon from emitting polygons. Higher values will increase the relative chance of that polygon getting an emission.

I don't understand the attribute.

Could you explain the attribute in detail?

I think what you're referring to is that you can specify an attribute to control emission.
for example, you could create a color ramp on a grid, use the grid as a particle source and then use that Cd attribute to control the rate of emission.

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