Ava has many design options available in the Customizer -> Styling options that help you control design of your website. Some of those options are available for pages / posts individually inside the “Design Options” box available for each page/post edit screen. Design options always overwrite the global settings when changed. Design options also contain a couple of different option than the global options such as “Customize Menus” and “Sidebars”.
For example, let’s say you have selected “Boxed” for “Content Holder Style” from the Customizer panel to be used as the global setting for your entire website. But a page you are working needs to be “Full width”, then you can adjust it for the page by using the “Design Options”