Answers for build 28175 unless stated otherwise. Mechanics are subject to change. I am unfamiliar with PoE’s terminology and will remain so.
- Modifier stacking is inconsistent, external databases are still outdated and often wrong. For an extended discussion of damage modifier stacking, see here.
- Poise is resistance to both. Also note that there are two ‘poise bars’ and I’m willing to bet almost all players miss the second one.
- Armour pen reduces enemy DR by a percentage. 50% arpen against 50% results in 25% DR. No overpenetration at >100% arpen (0% DR).
- Armour affects all damage types except special sources (drowning, falling, health costs for runes).
- Elemental DR multiplicatively reduces leftover damage after armour DR. In 26973, rings added to elemental DR. From 28175, they no longer do. If an attack does 100 damage and you have 50% DR from armour and 10% fire res, you have 55% effective fire DR.
- Bleed is physical. Dual Breaker does bleed/physical even with an ice gem/facet (and can shatter). Damage while moving scales with physical damage increases.
- From my testing status build-up is mutually exclusive outside of advanced status effects (fester, overload, searing). New overrides old.
- For bleed: Almost all attacks move enemies, even just back and forth. Involuntary movement is movement. Try an AoE knockdown rune on a weapon with bleed (Petal Spear’s Crush with a spike is strong). It is not Dark Souls bleed.
- Kickback multiplier is how far enemies are pushed back on hit. Does not scale with poise. The value is (or was) much increased for frozen enemies.
- Status/ailment buildup scales with damage. Some enchantments/effects (plagued chests, rare runes like Static) grant increased buildup.
- Crit is checked per hit and per enemy.
- Backstabs have varying numbers of hits but total damage is broadly even across animations and weapon classes. Backstabs cannot crit.
- Lifesteal is global, as are most stats. Conditional effects are mostly labelled as such (outside of the focus regen enchantment on helmets, which only works in combat).
- Lifesteal applies to most damage except reactive, DoT, and indirect damage. There were recent changes to this in build 28175, especially removing lifesteal from reactive damage.
- Runes with odd scaling like the one you mentioned (AoE shockwaves, stamina runes) are inconsistent. I have found no rhyme or reason to it. That said …
- Weapon damage is modified for attributes (see link above for an example) and used as the universal basis of damage calculations.
As for sBAM’s post:
- Elemental facets were changed recently to modify base damage, not final damage; so the 10% damage penalty holds at level 1 and is nearly meaningless at level 16. Should be about 2.5%–2% around levels 9 to 12. This makes facets with damage penalties more appealing.
- Attack speed seems to stack inconsistently. I only know few sources – % of durability, woven ring, after shock status, rapid, quick. I know of no database with sensible data on this (such as frame counting).