These two sections differentiate which information is flowing into NetGalley from your ONIX feed, and which information is NetGalley-specific.
If you are using ONIX and the manifest process to create titles in NetGalley,
When an ONIX update is received for an ISBN associated with an existing title record, that update will be pushed to the title record as well for the following fields: Title, subtitle, authors, imprint, pub date, price, currency, and format. For example, if the first ONIX record NetGalley receives for a title had the price set for $10 and the second had the price set for $15 USD, NetGalley will update the price to $15. We also update the description field, however, it works slightly differently. If a description has been edited manually on NetGalley, we will not overwrite the description regardless of if the text is different in the most recent ONIX record. But, if the description has never been manually edited, the description text will be updated based on the most recent ONIX record.
We recommend that you spend most of your time editing the NetGalley Extras section. All the good stuff is in there!
Once your title is already set up, the Basics section will appear under Edit Title Details. The Extras section will appear under Edit Marketing Information.