This new extension is a variation on the dynamic structured snippets that was released earlier this year. These extensions enable you to select the type of values, like amenities, product types, and brands, that makes sense for the campaign that you’re running. You can add the values you want included under that list, in any order you desire.
The difference between callouts and structured snippets, according to Google, is that callouts “highlight what makes your business or products or services unique,” while structured snippets should be used to “highlight a specific aspect of the products or services you offer.” The difference is pretty subtle, as they can be used together. Just make sure you don’t duplicate what you’re highlighting in both.
Both callouts and structured snippets have a 25-character limit. At bare minimum, you can show one structured snippet, and the maximum number of snippets you can display depends on the character length and screen size.
Check out the blog post for more details on the difference between callouts and structured snippets.