By default, menus will be displayed using your theme’s single.php template. This is often designed for your site’s Posts and is not appropriate for menus.
In the past, the Menu shortcode was used so that you could add your menu to a Page, which themes often display in a format more appropriate for menus.
Since version 1.5, Menus include an option to choose the template you’d like to use for your menu.
This will show you all the available templates you can use from your theme. They may include:
Default Menu Template: This is the single.php or single-fdm-menu.php template in your WordPress theme.
Default Page Template: This is the page.php template in your WordPress theme, if it exists.
Page Template: Any Template Name: If you theme includes additional Page templates, these will be listed with the Page Template prefix.
WordPress introduced special post type templates in version 4.7. If your theme includes any templates specifically for the fdm-menu post type, these will also be available for selection here.