Sub-Gallery is a gallery of one of the specific flexi post. It means you can add additional images for the primary images.
Eg. iPhone is image is posted and as a sub-gallery, the supporting images like front, back, edge images as a thumbnail at the side of main image.

To achieve this features, you must enable it from Flexi -> Extension tab.

After it is enabled, click on blue toolbox to have setting page related to sub-gallery page.

Add Sub-Gallery Button: This will display button after primary images is posted and while editing the existing post.
Button Label: You can change the label of the button. Eg. “Add supporting images”
Sub-Gallery Form: To modify the posted sub-gallery this page is used. Create a new page with shortcode as
[flexi-common-toolbar] [flexi-standalone edit="true"] and link this page.

Note: Specify location to display at detail page. If it is hidden it will not display the gallery.
…../wp-admin/admin.php?page=flexi_settings&tab=detail&section=flexi_detail_settings