Ok, I'm glad you brought that up as my language is obviously unclear. I'll go back and edit that to better clarify. What I meant to say was, the coins must be agreed upon at the time of exchange. In other words, you couldn't just decide to receive all bitcoin incoming transactions as Ethereum. You would be deciding to swap for one coin or another at the time of the exchange with the sender. Where as in ARK you can actually decide independent of the sender which coin you would like to receive. I can see where I was unclear, and I will go back and restate that the way I meant it to sound : ).