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Now what is the difference for a legendary to MERIT another legendary who is already a legendary RANKER on this forum
This is a little off-topic.
Legendaries who are active in signature campaigns needs few merits too because campaign managers would require or prefer those who received xx amount of merits in the last 120 days. If they don't meet such requirement, the chances of them getting accepted is lower.
In your case, it's about the bounty Campaign they are running, while I am talking about having Merits when you are already a higher ranked member in the forum just like legendary to legendary.