Engineer is a faction. Its bonus is Gear Upgrade: with 2 Engineer classes, battle items work one level higher in battle; with 3, manuals too. Every item has an Engineer level listed on its page, so items at max level go one higher too. Items lent by Peddler’s Pack are not raised. It works on your items, not on units.
Bonus by tier
Distinct member classes on the field
2 classes- Battle item level +1
3 classes- All item levels +1, manuals included
