Re: caustics and light multiplier
I think that we must use the decay due to the physical aspect about VRay.
About the difference to Spot and Direct, this is a "in depth" question, so I think you must ask directly to Chaosgroup.
Anyway, the spot light can "shoot" the photon only in the light cone. The Vray Light everywhere. This is why ofter it's better use a spot/direct light.