Sometimes, some genealogy data files contain links to records that don't exist in the file. All that is available is the link information. Since there is no record, there is no place to attach these references, so these are added into an Undefined Records section. This section is only included if there is at least one undefined record.
If your file has undefined records, you should attempt to fix these in your genealogy software. Maybe you need to define the note, source or object that the link is supposed to point to. Or maybe the link is simply pointing to the wrong record. You'll have to be a detective to figure out what's wrong so that you can fix it.
Undefined Record Numbering
Undefined Record numbers use the word "UNDEF" as their prefix and are numbered sequentially.