Using the TEA Party TApp as an example, the party-actor compiles to the actor that runs inside a hosting_cml.
The only thing that the back-end actor does is handle incoming requests.
This back_end_actor is different than the state_machine_actor which run inside the enclaves of the state machine replicas. Those state machine actors handle the queries and commands that directly interact with the state machine. In traditional web 2.0 applications, these are usually called Stored Procedures that run inside the database server.