Let us assume that an ESME has established 10 trx (or tx) binds (tcp connections/sessions) and submits a "submit_sm" PDU over one of those 10 binds.
Can we assume that the corresponding "submit_sm-resp" PDU "arrives" over the same trx (tx) bind; that is over the same tcp stream (connection/session) ? I believe that this is correct. For example, an ESME may "reuse" sequence numbers across all binds. That is, ESME may submit a "submit_sm" PDU over bind-1 with sequence number "123". In "parallel", this ESME may also submit another "submit_sm" over bind-2 with the same sequence number "123". So, if we assume that "submit_sm-resp" does not "arrive" over the same bind - stream, then there is no way to correlate request /response. So, the response must "arrive" over the same bind (tcp session/connection). A "DLR" may of course "arrive" over any of the established sessions - the same or a different one.