It seems it is not possible at the moment to have a decal not be applied to everything. That’s not nice.
I only need a decal to affect the landscape and it is not allowed to be drawed on other actors.
And that’s not possible at the moment, since there is nothing like a blacklist or whitelist for decals. Please implement this. It’s really necessary.
Second. I decal scorch marks onto targets hit with an explosive shell, and it’s really weird to see them bleed across the floor when the shot nicks an enemy in the foot and he starts walking.
You can already do that :). In mesh properties expand the rendering group and uncheck Receives Decals. Done 
The decals need channels better than this, if for example you have your character mesh, and uncheck the receives decals to no receive decals from walls, ground… but if i like put a blood or other decal in the character mesh then what ?
In addition, sometimes you want a decal to be applicable to two separate surfaces but only want a GIVEN decal to apply to one.
See my earlier point about scorch mark decals; I want to put the same scorch mark on any surface the round hits, meaning both the floor/walls AND the enemy skeletal mesh are valid targets, but if a decal is spawned attached to the skeletal mesh, I don’t want THAT specific decal to be visible on the floor/walls if the decal’d part of the enemy comes too close.