[lch]

There are different types of sources for boni in the game. One might come from a binary attribute which I would call "flags", apparently from Dom:PPP. Those are usually set by spells. Another source for a bonus comes from a bless. Then there are "abilities" which have some numeric value, this is usually what units have set and what magic items grant.

Those three things stack. The last source, "abilities" with a numeric value, stack on other such things that raise the ability. The first source is a binary flag and thus doesn't stack with other binary flags. That way different spells with the same effect don't give the same bonus twice. This is true for all kinds of boni in the game, not just elemental resistances.