It's a change to how trades are done in the background.
Before, the taker client would select the arbitrator, now this is done by the maker. Before this change, a rouge arbitrator with a modified client could have taken all open trades, assign themself to them and pay them out to themself.
After this change, the maker-side selects the arbitrator, and a rouge one would lose a bunch of bond in the trades they weren't assigned to. The more arbitrators there are, the less likely it gets that one of them could go rouge and make a profit. For example: at 20 arbitrators, a rouge arbitrator would be expected to lose the 15% bond for 19 out of 20 trades. That is on average nearly 3 times higher than the expected return of the 1 trade they can actually drain.
This is why we will ask many trusted community members to become arbitrators once the software has become more stable and there aren't multiple updates per month anymore.
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