View this topic to understand CL command processing.
If you choose i5/OS™ CL command processing and two connections, normal output returns on the first connection, and error output returns on the second connection. The REXEC server returns all spooled data that is written to the default printer file (*PRTF). This includes data that is written to the screen if the command is run in an interactive job. Any messages written to the job log return to the client on the second connection.
If the client specifies that all data returns on a single connection, the job log messages are returned first, followed by any spooled output.