Exact fill width,slic3r ignore the fill density

As the topic says,if i set Print Settings->Advanced->Extrusion width->Infill to an EXACT para(80% e.g.),the printer always infill regardless of the para of Print Settings->Infill->Infill density.
And if i set the para of Infill width to 0(default),it print well considering the para of Infill density.It really confuse me!
Anybody answer me?

That setting, if I'm following your pathing correctly, is for the actual width of each stroke during infill. That is not the infill "density," it is simply how wide each pass is when it is filling. Infill density is under the "infill" area in Print Settings. Expressed as a decimal out of 1 (ie. 0.4 infill is 40% infill).

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