Set Bonus

Set Bonus

"Set Bonus" - special temporary buff on your character that benefits from the items you are wearing. Buff is calculated from 7 items slots: Helm, Armor, Pants, Boots, Gloves, Weapon and Shield.

The greatest impact comes from the number of Excellent Options on your Items. Item Grade (Pad Low, Hades High), Item Levels and Luck Option are also taken into account.

Set Bonus affects the following characteristics: Physical Damage, Wizardry Damage, Critical Damage and Maximum Life.
Maximum values are not limited. As an example at 500 Set Bonus you will get +15% Damage & Defense, 4500 HP and 375 Additional Critical Damage. This are the values you will get by wearing Great Dragon Set+12+Luck+3 Exc. Opt. on each item.

Basically, every item, every level, every option will influence the power of your character as a whole.
Your current Set Bonus is displayed in the buffs panel and is updated instantly, when you change your items:

Sometimes items might give you better defense, but lesser Set Bonus and vice versa, so it is for you to decide what suits you best at the moment.

Source: https://forum.bless.gs/index.php?/topic/40736-set-bonus/