Case hovers work in all Lightning apps, including apps with standard navigation and console navigation. These handy hovers are specific to cases only. Case hovers work anywhere there’s a case number, except in list views.
You can’t customize the Latest Update section of the case hover. This section displays only one post. All post types are supported. For example, if the latest post was a post from a social channel like Facebook, that’s what displays in the hover. Pretty awesome, right?