Every attack has an assigned attack type, and every unit has an assigned armor type. Certain attack types perform better versus different armor types than others, and some won't even deal damage. For example, the Missile attack type deals more damage versus buildings. The multiplier for that is 1.15x, so you deal 15% more damage. Thus, an attack that would have dealt 300 damage will deal 345 (300 * 1.15). On the other hand, ballistic attacks deal considerably less damage versus aerial units, with a 0.5x multiplier. So an attack that would have dealt 100 damage will do 50 (100 * 0.5).