Hello Joe, the option for a preview exit is enabled in the OD Admin client when connected to the iSeries. It is in the Application on the Miscellaneous Options tab - and the help text says:
Use Preview Exit
For Multiplatforms, z/OS, and IBM i servers, this option enables the retrieval preview user exit for the application. If this option is selected, a user-written program is called for each document that is retrieved from the server. Any information that is specified in the Parameters field is sent to the user-written program.
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I'm doing some research to find out more.