Stacking refers to applying similar effects multiple times, e.g. multiple sources of regeneration or protection.
The basic rule of thumb as to whether two sources stack is: if they are the same effect, they don't stack, otherwise they do stack. Of course, that's quite ambiguous.
Attacks are attempted to be blocked in this order: Awe → repel → vine shield like effects → chance to hit → damage (try) → fire shield
(todo: clarification/examples on the rule of thumb)
Natural and armor protection stack with diminishing returns. The exact equation is Nprot+Aprot-(Nprot*Aprot*1/40), rounding to the nearest integer. Easier than calculating it is to just reference a premade sheet of calculations. The axies in purple are Nprot and Aprot and are interchangeable. The resulting prot values are color coded to aid intuition.
A slightly inaccurate estimation is to take the greater value of armor protection and natural protection and add half the lesser value.
Regeneration stacks in the following way;
Regeneration is based on the unit’s maximum hp meaning that hp-boosting items like Coral Sword will increase the regeneration tick. Similarly if a pretender is attacking into enemy dominion, their maximum hp will be reduced, thereby reducing regeneration.
Healing ticks from regeneration will not commence until current hit points are lower than maximum hitpoints.
No one knows how fear works. Not even Illwinter. Especially not Illwinter.
What we do know is that guys with a Fear Aura will regularly instigate Fear checks in random squares around them (the amount of squares is equal to the Aura value minus 1), forcing characters in those squares to pass Morale checks versus a certain value, massively impacted by their health & the health of everyone in their squad. If a character happens to fail the Morale check, they will rout, and their entire squad will rout if they are a troop instead of a Commander. Units who rout will then try to flee the battlefield through the side they came in from, fighting (very poorly) only if they have to get enemy units out of their way. Fear Auras stack, but I'm pretty sure that there's no way to increase the difficulty of the Morale check (other than reducing the target's Morale).
Resistances stack, but only if they're from different effects. Protection from Cold does not stack with Resist Cold, since both spells apply the same effect. Minor Cold Resistance stacks with these spells, however. Check the green text on a Resistance spell; if it's the same as the green text of another Resistance spell, their effects will not stack. Consider the stacking to be similar to the way Regeneration works.