When you have fully equipped with Equipment and all of them are above certain tiers, you will get Set Effects.

Your lowest Tier Equipment will decide which Tier the Set Effects will be.

e.g.)

- Helmet / Garment / Gloves / Boots / Weapon / Accessory Tier is 3 / 3 / 3 / 3 / 3 / 4 = Tier 3 Set Effects

- Helmet / Garment / Gloves / Boots / Weapon / Accessory Tier is 5 / 5 / 5 / 5 / 5 / 3 = Tier 3 Set Effects