This (standalone) is a gallery which is built only for site administrator.
This do not have any frontend submission form.
It cannot be managed from user dashboard and it doesn't have any layouts and settings.
It is used, if an administrator wants to add additional images as a gallery on the top of single flexi post. These images will appear on the 'detail page' as a sub gallery below/side of the primary image.
As reference to the screenshot which is taken when you edit the Flexi post, there is the option to add multiple images to display along with the primary image.
This image will automatically appear on 'detail page'.
But you can only display the set of images uploaded on this page wherever you like with the shortocode. That shortcode can be found at the right pane of edit screen.
Above image has shortcode eg. [flexi-standalone id="430"] which will display gallery as below screen shot.