ToME: the Tales of Maj'Eyal

My Solipsist has 20% all damage penetration from Crimson Robe. But I'm not sure how resistance penetration really works. Is it additive with my mind resistance penetration or multiplicative?

If it's additive I guess I can swap one of my resistance penetration items with better one to get 80% mind resistance penetration + 20% all damage penetration (so 100% in total).

But if it's multiplicative, it's quite useless stat for me, since I already have 100% mind resistance penetration and as far as I know any resistance penetration past 100% is useless. And even if I want to replace my resistance penetration items I'll only get 4% mind resistance from 20% resistance all if I remove 20% mind resistance item.

I was looking through wiki and forum but could find any inforamtion about all damage penetration (only add damage resistance).

EDIT: Nevermind, for some reason All damage penetration has separate stat, but it also increases every resistance penetration stat you have above 0 by 20% additively. All damage penetration stat itself has seemingly no effect on damage. At least my Solipsist in training room with 90% mind pen and 20% all pen hits 100% res dummy for exactly 90% of damage it deals to 0% dummy. While with 100% mind pen and 20% all pen damage is the same with any resistance.

I'd say what you see is the result of the character sheet already including alldam res/pen in the specific resist values displayed. As a result, the alldam res/pen stat only applies to any element that isn't mentionned already. Aka one that uses the "default" value because you have no gear/talent that gives that resist/pen in the first place.