Set Buff is a special Buff for your character that depends on what you are wearing.
Buff is calculated from 7 items - Helm, Armor, Pants, Boots, Gloves, Weapon and Shield.
The increase comes from the number of Excellent options, the Luck option and sharpening on your items, in general, each item, each level, each option will strengthen your character.
The Set Bonus is calculated as follows:
((number of items with LUCK option) + (sum of item level of all items / 3) + (number of EXCELLENT options * 5) + (sum of increased level of all items)) + ((number of items with LUCK option) + (sum of item level of all items / 3) + (number of EXCELLENT options * 5) + (sum of increased level of all items) * Class percentage)