In Mendix 5.17 microflow buttons were replaced with action buttons. See the release notes:
https://world.mendix.com/display/ReleaseNotes/5.17.0
An action button can not only call a microflow but also directly open a page. This is something you would need a small microflow for before. Finally, an action button can do nothing which is useful when you are setting up a page and you are not ready yet to define all behavior of buttons.