Damage calculation in battle.

Which specific point is being discussed in the reference? Give me some context, and I'll take a look at what's going on there.

In the new version, which you linked in your signature (accessed through a website), there are no specific numerical examples in the "Damage Calculation in Battle" section. However, one of the older versions had the following example:

20 angels with an attack of 21 attack 20 angels with a defense of 60,
+ the defending side has the "Armor Expertise" skill, and the attacking side does not have an attack skill, + a "Shield" spell of the "Expert" level is cast on the angels being attacked.
Damage calculation: Initial damage U1 = 20 * 50 = 1000 (1 angel deals a constant damage of 50), defense is greater than attack by 60-21 = 39 => damage U1 will be reduced by 39 * 2.5% = 97.5%, but the maximum can be 70%, so U2 = U1 - U1 * 70% = 300, the hero has the "Armor" skill => U3 = U2 - U2 * 15% = 255, then we take into account the "Shield" spell U = U3 - U3 * 30% = 178.5, or after discarding the fractional part U = 178 - the resulting damage.

Of course, this is a rather rare nuance, but it will become noticeable when the defense advantage is 23 or more. And if we take into account that the unit may already have the "Stone Skin +6" spell and is defending, then for the Swamp faction, this scenario will not be so rare, and the hero may also have a "Titan's Shield."
In general, since the Swamp is a defensive faction and often has to take the first hit, I believe that one of the main points in its tactics is to constantly estimate the damage that it is about to receive, while for many other factions, it is more important to know how much damage they will inflict first.