Email metadata confusion: Headline vs. Subject field

After years of using DTPO, I’ve decided to try out the email archiving capabilities. I installed the Apple Mail plugin and archived a mailbox from Apple Mail. This worked great, but has led to some confusion about the metadata that DTPO pulls from emails.

In the emails imported using the Apple Mail plugin, DTPO is correctly pulling the Date Created, From, To, and Subject fields. In addition, DTPO appears to be showing the Headline field. But the headline only gets shown for about 80% of the emails. The other 20% are blank. For example:

Screen Shot 2017-12-28 at 10.24.57 AM.png

Any idea why DPTO leaves the Headline field empty some of the time?

To complicate things further, the metadata is different when I don’t use the Apple Mail plugin. Before trying the Apple Mail plugin to archive entire mailboxes, I stored very little email in DTPO. I occasionally dragged individual .EML files from Apple Mail or Finder into DTPO. Doing this, I have found that the Headline field is populated about 70% of the time, whereas the Subject field is closer to 50%. I can’t figure out the reason for the inconsistent treatment of emails. As you can imagine, it is very difficult to search for emails when the subject is sometimes treated as a Headline, sometimes as a Subject, and sometimes as nothing at all.

Any ideas what is causing this? What is the difference between the Headline and Subject? Why would email subjects sometimes be treated as one and not the other?

This is with DTPO 2.9.16 on High Sierra.

After submitting my question, I made a discovery. Of the emails imported using the Apple Mail plugin, those that are replies or forwards (i.e., whose subjects start with RE: or FW:) are the emails with Headline fields. The “new” messages that are not a reply or forward do not have a headline.