All spells and attacks in Dominions 5 have an area of effect property, hereafter referred to as AoE, which determines how many (and sometimes which) entities they affect.
Weapons always target a single entity unless an AoE of 1 or more is specified.
Additional effects on weapons function the same way, and will either hit the same entity as the original weapon attack if no range property is specified, or the listed AoE, calculated from the square where the initial attack landed.
Generally encountered on self-buff spells, an effect with AoE caster only affects the unit that cast the spell, and cannot miss.
In the case of start of battle and auto spell items, they will affect the wearer of the item.
If a communion master casts a spell with AoE: Caster, its effects will also be applied to all active communion slaves. This includes the aforementioned effects triggered by items.
Generally encountered on offensive spells, an effect with AoE: One person will hit a single entity from the square it targets. This obeys normal projectile targeting rules and does not necessarily have to be the original target of the effect.
An AoE of 1, encountered on all kinds of weapon effects and spells both offensive and defensive, affects all entities in a single square of the battlefield. Thus, up to 6 size 1 targets sharing a square can be affected, as well as all other combinations that fit a single square.
In the case of additional weapon effects, the targeted square will be the one containing the unit hit by the triggering attack.
And AoE of 2 or higher means that the spell or effect will target a total number of squares equal to the AoE value. For example, an AoE 9 spell will hit 9 squares on the battlefield, although the distribution is based on RNG and does not have to be even, so the final area that it affects will not necessarily be a 3x3 square.
The initial target of an effect is always a single square as defined by targeting AI and precision. From the initial square, targeting follows this logic:
WIP - clarify inspector info vs in-game
Combat spells that are listed as "Enchant Battlefield" affect the whole battlefield during combat. They can be persistent effects (shown in the upper right corner while active over the course of the fight) or one-shot effects that attempt to apply an attack, effect or buff to the entire field.
Persistent damage effects each have their own formula that specifies the damage, frequency, targeting mechanics and individual AoE of its particular attacks, though these are often not detailed in-game.
These spells will generally contain additional targeting restrictions such as only affecting enemies, allies, magical creatures, etc.