Slitherine

I was recently wondering about some types of damage reduction. Some of the definitions are clear for me, according to the wiki there are those types of reduction:

'Ranged', 'hero', 'invulnerable', 'city', 'feel no pain' and 'witchfire'.

Ranged and witchfire are obvious, since they're referring to the concrete types of attack. And according to wiki, rest of them (hero, invulnerable, city, feel no pain) are just different names of reduction of all kinds of damage. And while names "hero" and "city" are rather self-explanatory (hero - for hero type of unit, city - for being in the boarders of city), but last two... I don't get them. How "invulnerable" can referr to something which only partially reducing damage? This word suggest total immunity. You can't be partially pregnant or partially invulnerable. Moreover - do I understand correctly that "Feel no pain" damage reduction is something like "invulnerable" reduction, just with different name?

And most importantly - how is it calculated when some unit has more than 1 type of "all attacks" damage reduction? how there is calculated total reduction, if for example Chaplain has 50% hero damage reduction AND 50% invulnerable damage reduction?

Many thanks in advance for reply.

The reason Invulnerable and FNP exist seperately is due to how the original game works. You can have Invulnerable Saves (from things like force fields) that work like armour but ignore armour penetration, and then FNP is a chance to ignore damage you receive. In Gladius the significance is mostly that you can get both at once, from different sources.
Different types of damage reduction should stack multiplicatively: 50% hero + 50% invulnerable should result in 75% damage reduction. I think I remember reading somewhere that there's a cap, though.