Hi Carrara,
When LEAVE TO TRANSACTION is used, the execution control will leave the calling program and it moves to program of the Transaction. So, control cannot come back to the calling program.
In the CALL TRANSACTION, the control remains the called program and executes the called transaction from the called program. When user comes back from the transaction, controls leaves execution of transaction and remains in the called program. So, he can come back to the program.
Regards,
Vijay