Polkadot Architecture Improvements
Asynchronous Backing (Contextual Execution)
Hierarchy of Relay Chains
If the token transfer is carried out in conjunction with SPREE modules, then that ensures that so long as the parachains execute correctly, tokens can only be created and destroyed in an agreed upon way. In turn the correct execution of the chains code is guaranteed by the availability and validity scheme. SPREE ensures that shared code needed for the token transfer logic is correct as well. Even though chains can change their own code, they will not be able to change the code of SPREE modules. Instead the code of SPREE modules is stored centrally and the execution of that code and its storage will be sandboxed from the rest of the state transition. This ensures that this token transfer message is interpreted correctly and obtains the guarantees about tokens we want.