Scale this to determine how far the particle falloff begins and ends. Do note this should be modified very little and kept small as if you make it too large you can have the decal appearing beyond the bounds of the surface are wanting to apply the decal to.
Determines how soft the edge is for the falloff radius. For example you would want it to be max for something like a poster where you want it to go to the edge of the image input but for something like fire you want to fade the edges off so it looks like it covers an area.
Vertex Tex Alpha Toggle
This is a setting that should be on for particles and off for static meshes. If left on for static meshes it will mess with the transparency of the decal.