For multiple session from same SAP instance (connection), a user can interact with a particular session through the use of parameter 'session number" of certify action "Set Instance".
Please use parameter 'session number" if you there are more than one session on client screen from an SAP instance and need to interact with any particular session other than the first session. By default, if no session number specified by the user, then this action interact with the first session.
The action with session number=2 would fail if there are no second instance of the SAP system.
Note: SAP learn screen section "Available SAP connections" displays connection information about different SAP connections. (Open SAP Learn utility and go to Tools->Diagnostic Tools)