Generally, the reason for there being an UNDEF report is because a job is being processed through Deliver, but there is no corresponding report definition for a DD that matches the Deliver criteria.
As a result, Deliver processes the report but gives it a name of UNDEF, as it does not know what to call it, due to no matching report definition.
Some clients regularly will look for UNDEF reports, when new jobs or new reports in existing jobs are run a first time, as a sign that something is not defined properly.
A STACK report is a section of a report, depending on if Text Identification is used.
If there is no Text Identification defined for the report, then the result will be the whole report.
It is best to review any Job's definition that contains new requirements, to make sure that things are defined properly, including reviewing the JCL in question, to ensure that the particulars match.