Is armor's DR additive and covering whole body?

I must be missing something obvious here, but the costumes seem to provide MUCH better DR than even the best armors: The Silver Shroud costume initially gives 42 DR (both normal and energy) while i.e. Combat armor chest piece only 15 and CA leg - 8 DR.

If the total damage resistance from mix-and-match armor is the sum of all its parts and it doesn't matter where you are hit (leg with 2 DR or chest with 20) then those are comparable - full, unmodified (and much more expensive) combat armor would give 59 DR, but if only the DR of hit body part is taken into consideration, then costumes are hands down better option.

So, which one is it? Is the DR of mix-and-match armor the sum of DR of its parts ? If no, whats even the point of taking such armors over costumes?