Damage reduction reduces damage from each source:
Consider having DR2, if you were attacked by 6 sources each dealing 3 damage, you would take 6*(3-2)=6, however, if it was 1 source dealing 18 you would take 1*(18-2)=16.
My question is how to determine the number of sources of damage in regard to DR when dealing with "AOE-ish" situations. Consider the two following scenarios:
A classic trap of a moving wall that crushes you (to the wall in front of it).
- If the wall is blunt, it deals all its damage as one object and can be considered a single source and so DR applies once.
- If the wall is spiky, then you can argue that the damage is distributed by each spike, thus DR applies multiple times. In this case, you might also be able to argue that a creature of size tiny (or whatever is small enough) would not get hit at all since it can move between the spikes (and the wall can't close on it all the way). You could not make this argument if you treat the wall as a single damage source, or could you?
A classic (densely) spiked shallow pit (shallow enough you could stand inside it).
- You stand in the pit with both feet and the trap can be considered a single damage source.
- You partition the pit between your feet so that now you have 2 pits, with one foot in each. Is it now 2 damage sources so that DR applies twice?
I'm not saying you should treat a sword as 10^20 atoms each dealing its own damage, but would you just treat "a trap" as "a single source of damage" regardless of how complicated it is?