What is the difference between the 'line_item' field, and 'podcast_title' field?

The differences between these fields is based on the ad placement type

'Line_item' is the name of the DAI (dynamic) ad placement. If 'line_item' field is blank, this means it is a BI (embedded) ad placement, and the title of the podcast will appear in the 'podcast_title' field. 

For DAI placements, we often do not receive podcast title information, so the 'podcast_title' field is often blank unless the placement was BI.