@Xolos is right but I’ll try to expand on what he said a bit.
People also use the terms “additive” and “multiplicative” in referring to the same thing, somewhat sloppily.
In his example the two +20% damage buffs are “additive” with each other but would be “multiplicative” with e.g. a splash damage buff.
The reason people say “multiplicative“ buffs are better is because in this game if you have a choice between stacking buffs of a single type and getting a few different “types” of buffs, the numbers generally favor the different-types approach.
Building on the above example, say you have a rad gun that deals 1000 damage.
Which is better: 3 different buffs of 20% to gun damage, or a 20% buff to rad, a 20% buff to gun, and a 20% debuff on an enemy?
One might think it shouldn’t matter, It’s three 20% damage increases either way. But in fact the different-types option will be unambiguously better because of how BL’s engine calculates the damage dealt:
Damage = Base x (1+gun buffs) x (1+elemental buffs) x (1+debuff)… (Note, greatly simplified).
Damage with gun buffs = 1000 x 1.6 = 1600 (The three gun buffs are “additive”).
Damage with gun, rad, debuff = 1000 x 1.2 x 1.2 x 1.2 = 1728 (these buffs are “multiplicative”).
Because the additional gun buffs aren’t as effective as stacking 1 gun buff with a rad buff and an enemy debuff, people would say the additional gun buffs have “diminishing returns.”
So the point here is that it is generally better to spread damage buffs out across different types - elemental, splash, gun, enemy debuffs, etc. If you already have a ton of gun damage buffs, probably go get more elemental or splash bonuses instead of still more gun damage.
Of course, if your choice is between a really big “additive” buff and a small “multiplicative“ one, the math may well tilt in favor of the big additive buff.
For your purposes, SV probably doesn’t have a major “diminishing returns” or “additive” problem if you’re already reasonably balanced between gun, elemental, and splash buffs.
I admit I don’t use SV much, but that’s mostly down to personal preference. It may well be right for your build and gear.