Thanks for your questions.
The default orderby clause for [mla_gallery] has two values: first, menu_order Ascending and second, ‘ID(post ID) Ascending. This displays items in the order that they were uploaded to the Media Library, i.e. oldest item (lowest ID) first, unless you override that with an explicitmenu_order`.
The menu_order field is a WordPress core feature and is available for posts, pages, attachments and all custom post types.
I am marking this topic resolved, but please update it if you have any problems or further questions regarding the above information. Thanks for your continued interest in the plugin.