I'm a little confused on armor stacking...going off of this image, one could cover the breaks in armor coverage with a cloak or coat, so I usually do. However, I've been noticing messages indicating that the attack penetrated said cloak even when the area should be covered by, say, the breastplate.
How exactly are the two sides of the armor vs. weapon check determined? Does it factor in all valid armor pieces present on the body-part (i.e. cloak, breastplate, chainmail, vest, shirt), does it select only one armor piece at random, or is there some other pattern I'm missing?
Armor is set to cover a certain area a certain percent of the time.
If a cap is set to 50% (I forget if it is, but that sounds about right), than 50% of the time it will be counted in the attack/defense step. So, a helmet covers closer to 100% than a cap (I think that percent coverage caps out below 100%, so nothing will always cover you), and can rely on an iron helmet to protect you more often than a steel cap.
Your breastplate actually has a fairly low coverage rate, but is quite thick.* You can find the data in the file item_armor.txt
[ITEM_ARMOR:ITEM_ARMOR_BREASTPLATE]
[NAME:breastplate:breastplates]
[ARMORLEVEL:3]
[UBSTEP:0]
[LBSTEP:0]
[SHAPED]
[LAYER:ARMOR]
[COVERAGE:100]
[LAYER_SIZE:20]
[LAYER_PERMIT:50]
[MATERIAL_SIZE:9]
[HARD]
[METAL]
All available armors are rolled for protection, but only the first selected (outermost armor that will offer protection) will be listed as defending.
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UB and LB step describe how far up or down coverage (and layersize, which determines the total clothing you can wear on any spot) extends. Your breastplate only covers the breast, in other words.