A 7-Day Deal is a time-bound, promotional offer where an item is featured for up to 7 days on the Amazon Deals page. 7-Day Deals will also receive a “Limited time deal” badge in search results, driving more visibility to your deal and brand. You can create both 7-Day Deals and Lightning Deals on eligible products in the deals Dashboard in Seller Central.
These deals are created to help sellers get rid of excess Inventory & boost their product rank rapidly.
Note: It may happen many of your products appear eligible for Lightning Deals, but none are showing for 7-day deals because this program is still rolling out.
Reference:
Eligible products(Login required)
Deals Dashboard(Login required)