That is not implemented yet, there is still only one arbitrator, so who knows, it might even be changed from a current plan. Here is the quote from the FAQ page
I don’t know exactly the answer to your questions, but I assume that an arbitrator deposits money to a senior arbitrator or a multisig of senior arbitrators, which I see are the arbitrators personally invited in the bootstrap phase. I also assume that they will buy some DAO coins and burn them or split the BTC between themselves. I know this would rise some decentralization concerns, but this is the best I can think of. The bitsquare is an open source project after all, so a fork in case of controversy is always a possibility.
The planned decentralized arbitration system was not implemented yet. I am the arbitrator as it is not safe to open it up yet. With the DAO we have found a good solution how we can open and decentralize the arbitration system (secured by a high security deposit of DAO tokens). As soon we get the final review from our lawyers we will publish the paper…