While to transfer the data from the through if any errors occurs until the errors are the complete the data is not transfer to the SAP system.
The system compulsory shows the errors. that errors are stored into the error logs (Transaction is SM35).
so the session method should not return any value.
In call transaction method data is directly pass to the SAP system.
So its compulsory return the value.
Because of the call transaction is the function.
A function should return the value mandatory.