At the top you can specify if you’d like your search results to include Open, Snoozed, or Done items.
If you select only Open items, the number of items matching the search will appear in the sidebar.
Dispatch lets you search all of your messages using most of the same criteria that’s available for rules.
The main section of the Search Modal lets you specify search criteria, just like in the Rules Modal.
This is how you save the search for future use.