Banners used to be just callouts with no margin, no border and no radius, hence the similar code.

They are meant to be used on top of pages, outside the main content, flush to the viewport.

They can be customised the same way as callouts, although the HTML structure differs (see the knobs on the side to add a title, a button, make them dismissable or use a custom icon).

In additions, banner callouts have an option to add a link near or in place of their button.