An 'undefined' page is any page that has been linked to from within Confluence, but that does not yet exist.

Undefined Page Linked From
Mapping source and target types to copy data to and from a modeling tool Page: Copy Data with DHTrace operation (Cameo DataHub 19.0 SP1)
Page: Copy Data operation (Cameo DataHub 19.0 SP1)
Page: DataHub Operation drop-down list (Cameo DataHub 19.0 SP1)