Sometimes an attack is not as easily outlined as a sword strike. What happens when a boulder or some boiling oil falls upon the battle field? What about those mythic dragons and their fire breath? More realistically, what happens when that musket or cannon misfires and explodes sending shrapnel all around?
The answer to these and other questions is the Area of Effect rule. This rule is modifiable by each attack using it, but for the most part follows one of two rules.
Equal Impact - this form of Area of Effect presumes that the entirety of the attack will be the same across the entire are. Things like falling boulders, felled trees, caltrops and so forth fall into this category. Simply measure out the radius or other predesignated area (some even suggest templates, which are great!) and lay it onto the field. Wherever the template covers is affected. Any player models underneath suffer the full damage of the attack. If a player's model is less than 25% within the area, they may attempt to use Acrobatics or some other Aptitude to dodge out of the way, otherwise they are hit. Simple and easy.
Point of Emission - this form of Area of Effect presumes that there is a point where the effect is stronger than another point. Things like fires which may be hotter at the core, explosions which may deal greater damage the closer to center yet trail off the further you get from the center, and rushing water and the like are all examples of Point of Emission. The template is laid out appropriately, and depending on the attack or the arbitrator, the damage effect is lessened every inch on the game table or every 5 feet in game from the Point of Emission. While this may vary depending on the strength of the attack, a good rule of thumb is to drop off 1d6 of damage effect per 1" from the Point of Emission.