Every admin menu item is protected by user capability. ‘Posts’ uses ‘edit_posts’, ‘Settings’ uses ‘manage_options’, etc.
If you don’t know what capability is required for access to some admin menu item, like mentioned ‘Customers’ use “Admin menu access” add-on in order to find that capability.
You may activate this add-on and open its dialog at the “User Role Editor” for the ‘administrator’ role, which has access to all admin menu items. Then scroll menu items list, and look at the “Capability” column.