christmas set is a set that you cant buy now (rare items sold out) that gives 0.5 progress boost every time you claim a reward from the progressbar and the 600% means that you can buy up to 600% boosts from the second page of the feature shop
azencor, i find the christmas set lvl3 is the most useful 0.5 progress which means it never goes down plus buying the progress boost is the most expensive
using lvl 3 christmas gear from level 157-158 while pressing the button really fast i only lost 10% boost for exp and actually gained 10% coin boosts so if you can click your mouse really fast boosts can be really proffitable
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