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

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.

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