Hi there!
TL:DR: My testing (and game experience) suggests that there is a "sweet spot" at 3 armor difference between the tank and the dps (rouge for example). If it is in favor of the dps then the enemies do not switch target in melee to hit the dps instead of the tank. 2 armor difference also works since in most cases you have enough time to kill most enemies before they would switch to the rouge. If you have other experience or some kind of manual to aggro please share.
Long version:
So I have spent a good amount of time to search for "how aggro work" in pillars of eternity. I have found everything about "Engagement", and that's all nice and dandy. However actual "aggro" seems to be less discussed. If anybody have a good link or already know please help out a fellow player .
I did some testing on a "skuldr whelp", three "forest trolls", two "forest lurkers". So here is two very basic scenario:
Scenario A:
1. Fighter (armored to the teeth, carries an actual "larder door") goes in and engages enemy.
2. After engagement along comes a rouges in light armor and shivs the enemy in the gut.
3. Enemy promptly forgets the fighter ever existed and happily stomps the rouge.
Scenario B:
1. Aforementioned Fighter jumps on three enemies. He has an engagement limit of 2.
2. Two enemies got engaged by him and stays next to the fighter.
3. Third enemy goes on to pick somebody from the party. Or not.
This means there is an aggro system here. And my testing show it largely dependent on armor. I left the fighter with huge deflection (61) with no damage (unarmed) and my rouge with big damage (30-40 hits with sneak) and next to no deflection (9). Hard mode.
I did the following with a lot of combination of armor variations and combinations of the aforementioned scenarios and enemies. This is the result:
1. Fighter naked, Rouge in 5 armor -> enemy stays on OR switches to fighter even if rouge engaged first. All the time. Every Time. No exceptions (ok there were some cases like 1 in a 100).
/*Engagement number doesn't seems to matter here. If the fighter goes in butt naked, and everybody else just follows after "inital aggro", and wears something, enemies will not leave the fighter.*/
Lets fine tune it:
2. Fighter in 5 armor, Rouge in 6 or 4 armor, fighter engages -> enemies stays on the fighter for one hit THEN switches to rouge. Sometimes even switches back to the fighter, sometimes not. This is weird as hell, didn't managed to find out what happens here. Either way 1 armor difference is inconclusive. Lets move on.
3. Fighter in 5 armor, Rouge in 7 armor, fighter engages -> enemies stays on the fighter for 2 or 3 hits THEN switches to rouge. Sometimes they do not switch at all. It might have to do something with the level of the enemy. In this case forest trolls never switch or just after 4-5 hits but forest lurkers switch after 2 hits.
4. Fighter in 4 armor, Rouge in 7 armor, fighter engages -> enemies stays on the fighter and almost* never switch to the rouge. EVEN if the fighter is way over the engagement limit! If I add a butt naked third character to the mix one forest troll will go and engage that sorry bloke. But if its just the two of them (rouge in heavy armor, fighter in light), and fighter got everybody in melee range, they never switch to the rouge.
However in the meantime something peculiar happens in the logs:
The logs says that enemies do engage the rouge (whatever armor combination), BUT they switch back to the fighter before they actually hit (there are not turn animation either if you stand behind them). This is interesting I do not know why it happens.
That's all folks. This made many fight quite easier for me (especially with enemies with high resistances). My dps might be a tad slower in armor but you don't "have to" hit with ever cc to stay alive.