Attack/Defense bonus for units based on the hero's specialization.

It adds 5% of the skill's percentage value. In other words, with an expert-level Necromancy skill, Isre gains an additional 50% of 30% at level 10, resulting in a total of 45%. At level 20, it would be 60% in total. The same principle applies to specialists in logistics, defense, attack, and navigation.

I don't remember the exact formula for unit specialists, but it should be correct in the physics section. In general, their bonuses are quite insignificant.