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In exchanges where both Steem and SBD were available, the total of both currencies did seem to be enough even for mass withdraw, so it seems they have traded between them indirectly. It is also important to remember that the calculation does not take account already fulfilled orders where the coins are sitting unused in user balances. In these cases the exchange might keep the coins in deposit wallet or even some third unknown wallet.

It basically measures the largest single withdrawal the exchange can handle. It doesn't really matter if it is hot or cold wallet.

If there is multiple concurrent withdrawals where the total amount exceeds the balance of specific withdraw wallet, the exchange can safely split them among multiple wallets.

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STEEM 0.04
TRX 0.33
JST 0.083
BTC 62069.28
ETH 1614.34
USDT 1.00
SBD 0.45