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

ps. It's also interesting about the Logistics and Necromancy skills - the description says that 5% is added to the skill for each level, meaning if a hero has Necromancy maxed out to Expert, they get 30%, but since the maximum value cannot exceed 100%, does the hero only need to level up to 14 to reach the maximum value?

5% is added to the percentage value of the skill. So, with an Expert Necromancer like Isre at level 10, 50% is added to the 30%, 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 engine. In general, the bonuses they provide are quite insignificant.