Manage Sites Storage Page Explained

You can see which which types of file storage you want to use for your site under "Manage Sites" > 'Storage",

Select the site from the "Select for a Site to manage" drop down menu, click "Manage'.

Click on the "Select file storage service" drop down menu and select the service you want to use. You can choose "Intranet" Files, Salesforce CRM library, or Box.

To use Box as your storage, you will need to connect to your Box account from "My Settings" > "External Apps".