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The possible fogModes are "dense", "linear", "linear_lit", and "exp".
linear_lit is the default. The fog fades linearly until 129.8 meters from the camera, at which point it vanishes. This is used to keep the fog from affecting the skybox (recall that skies usually have a farClip of 130).
linear is like linear_lit but without the 129.8 meter cutoff.
dense is algorithmically different from linear but tends to look visually similar to linear in practice.
exp is used when the party is underwater. I believe the opacity of the fog at a point is exp(distance*fogDensity), where distance is the distance from the camera in meters and fogDensity is the fogDensity field of the WaterSurfaceComponent. It is unlikely that you want to use it on SkyComponent or FogParamsComponent since it ignores fogRange.