Full set or mixed armor?
A full set activates a quality-scaled bonus, using the lowest-quality matching piece. A mixed loadout loses that bonus but can combine four role-specific piece buffs, such as Nerd Goggles for XP and Rogue Hood for loot stage.
Why does the lowest quality matter?
The lowest-quality item in a complete four-piece set governs the strength of its set bonus.
Are cosmetic slots included?
No. Clothing and cosmetic appearance do not change this armor calculation.