As far as I know, MultiDecalLevel specifies over how many additional adjacent individual surfaces a decal can span (happens when you place a decal on a nonplanar surface).
Set MultiDecalLevel to a higher value for large decals (like the Redeemer scorch mark, NuclearMark, with MultiDecalLevel 4) and to a small value for small decals (like the Enforcer shot, Pock, MultiDecalLevel 0).