The final sections on the Details tab for paperbacks and hardbacks are the Publication Date and Release Date.

A screenshot from the KDP publishing process. 

The first heading is 'Publication Date'. There is a selection option for previously published books.

The second heading is 'Release Date' and there are options to release the book for sale immediately, or on a scheduled date.

Publication date
You will only need to enter a publication date if this is a second (or later) edition. For first editions, the publication date and the release date are the same. 
If your book has previously been published, you must enter the date the books was previously published.

A screenshot from the KDP publishing process. The heading is 'Publication Date' and the 'previously published' selection has been chosen. There is a box in which to enter the date the book was previously published.

Release date
You can choose to release your book immediately (following Amazon’s review process) or schedule a future release date, up to 90 days in advance. Scheduled releases are not available for books that have been previously published.
If you schedule a release date, you will not be able to update your manuscript within 5 days of that date. Ensure all final checks and edits are complete before that point.

A screenshot from the KDP publishing process. The heading is 'Release Date' and the 'schedule my book's release' option has been selected. There is a box in which to enter the release date in GMT, and a link to convert this time to different time zones.

Find out more about scheduling a release date here: https://kdp.amazon.com/en_US/help/topic/GDTMGBCWHAY35NK4