'Exactly Matches' is used in a 'Merchandised Search' when the merchant wants to control the order of products displayed on search results for the exact word or phrase in search queries.
For e.g., the phrase 'New Arrivals' is set to 'Exactly Matches' in the 'Merchandised Search' to only show products that have been introduced in the last 7 days.
When a visitor searches using the exact phrase, the search results would only contain products that were introduced/added in the last 7 days.
However, when the visitor uses only part of the phrase that has been merchandised, the results would not match the 'Merchandised Search'.
'Exactly Matches' is used in 'Search Redirects' when the merchant wants to direct the visitor to a URL for any search query that has the words in exact order.
For e.g., the word 'Gift card' is set to 'Exactly Matches' to be redirected to the URL https://example.com/giftcard.
When the visitors search for the exact phrase 'Gift card' then they would be redirected to the URL https://example.com/giftcard. If the search query does not have the words in the exact order, the search results would not follow the redirect rule applied to the query 'Gift card'.