I read about the XCM post and the answer by Joe here. Now I wonder right now what is the actual way that current parachains adopt to xcm assets. According to their "Trust level", I think they may adopt "reserve backed transfer". If so, if it is possible for the asset that is transfered from Parachain A to Parachain B can continue to be transfered to Parachain C? To be specific, if Karura, Bifrost and Moonriver all open xcm channel with each other, can $KAR on Bifrost can theoretically be transfered to Moonriver? And if it is possible, whether it is actually $KAR transfered from the account on Karura controlled by Bifrost to the account on Karura controlled by Moonriver in the background? So my understanding is the xcm transfer involves 3 parachains, the path should be 1. Bifrost burn the sender account's derivative $KAR value and 2. Karura transfer the actual $KAR from the account controlled by Bifrost to the account controlled by Moonriver and 3. Moonriver mint the derivative $KAR to the receiver account on Moonriver?
You have it spot on. If Karura, Bifrost and Moonriver all open xcm channel with each other, $stKAR on Bifrost's sovereign account on statemint can be transfered to Moonriver's sovereign account on statemint and then Moonriver's soverign account can transfer those back to Karura's sovereign account completing the circle. At each point whichever chain's account has the tokens will mint some derivatives to represent them on their local chain and then burn them when they reserve transfer them on to another chain.
I say $smKAR rather than $KAR because $KAR only exists on the Karura chain. Karaura would have to mint an asset on statemint that it could then reserve transfer around (while never actually leaving statemint). This would be fine as the Karura parachain would be trusted to have the power to mint $KSR and so would be trusted to have mint rights to $smKSR. I.e. Karura and statemint trust each other enough that they can effectively teleport. (Maybe a parachain should be able to teleport any asset that they are able to mint on statemint of to make this simpler? I shall confer on this...).