Beneficial for advertisers to gain more control over what text shows in the structured snippets in text ads, Google is releasing the extensions as they had feedback of many users wanting to customize the information that shows in this format.
It is a variation on the dynamic structured snippets that came out earlier this year. You will be able to select the type of values such as product types, brands and amenities.
You will find great benefit as you are including the values you want, in the order you want.
Google explains to use callouts to “Highlight what makes your business, products, or services unique” while structured snippets should be used to “Highlight a specific aspect of the products or services you offer.” They can be used together very well but try not to duplicate what you’re highlighting in both.
Both have 25-character limits. As few as one structured snippet can display, and the maximum number shown depends on the character length and screen size.
For more on the differences between callouts and structured snippets, see the Blog post.