Sorry, been too busy to get back to this!
As some have mentioned already, in the aforementioned games, armor is ablative. Once it gets breached, the internal systems can be damaged, 1 point of damage per box/circle. When a system has all of its boxes/circles filled in, it’s destroyed. (Basically.)
The thought was that a system like this could be used to have a system in which PCs & major NPCs would actually lose capabilities as they get injured.
How many internal points? That would depend on the capabilities in question. The more power, the more boxes.
“Mooks” would have armor just like major characters, but FAR fewer internal boxes. IOW, glass cannons.
Both Spire: The City Must Fall and Heart: The City Beneath have a system that kind of works like this.
In each game, the PCs have 5 different types of Resistances, which are essentially the kind of damage they can take. So your Blood Resistance is a measure of how much physical harm you can withstand, with similar Resistances for Mind, Silver, Supplies, and the like.
When a PC takes damage, they accrue Stress to one or more of these Resistances. Every time they take Stress, the GM rolls to see if they suffer Fallout; he rolls 1d0 and if he gets under their current total Stress, they suffer Fallout. That's where the narrative Stress becomes some kind of concrete effect; they get shot in the gut and are bleeding, or they break a limb, or they're stunned, or their primary weapon breaks.
It's not an exact match, but functions similarly mechanically. Having played Spire a bit, I can say it's engaging and creates the sense of mounting danger, and the idea that at any point, something can go wrong for the PC.