In order for the exchange to process deposits and withdrawals, they must have an actively running instance of the coin's daemon, which is connected to the primary chain of the altcoin's network (i.e., not on a fork).

Wallet maintenance implies one of several possible things could be happening:

• The exchange wallet is not on the correct fork, and needs to be synced to the proper chain.
• The wallet has been updated by the developer and the exchange is in the process of implementing the wallet update.
• The wallet daemon on the exchange's server has hung or crashed.

There are likely other possible scenarios, but the general story is always the same: an exchange with a wallet marked as "maintenance" does not have an actively synced, running client of the altcoin available to the customer at that time, and has disabled access to the wallet to prevent customers from placing requests which cannot be completed.

Because the amount of each customers' funds is kept in a database hosted by the exchange, they are able to process trades for a coin despite the wallet being in maintenance.