1. The exchange have 1 central wallet for each coin.
2. Once you sign up & deposited, the coins goes into the central wallet for that coin, & your account is updated.
3. When trading, only records in account is updated, no real coin movement.
4. When you withdraw, the coins is transfer from the central wallet to your wallet.
Regards.
1) Untrue, at least with cryptsy. (Read 2)
2) when you deposit for example bitcoins, you have to create your own private deposit address at cryptsy. When you check the account balance of your deposit address using any block explorer, you see your balance which means the coins aren't moved to a central wallet.
3) this is ofcourse true
4) coins are sent from your deposit address as you can see in the transaction history
My guesses is based on how a forex exchange worked. Not sure how cryptsy works. Bter creates an address for every type of traded coin, for every account. Highly not cost efficient.
Regards.