A fragment identifier is part of a URL that points to a specific area on the destination page. When a browser loads the URL, it will automatically scroll to that area.
The customer order page uses a fragment identifier for each ticket group. This means that a URL with that fragment can link to that specific ticket group–which is useful when there are a lot of groups on the page. This is called the "Order URL" for that ticket group.
In Box Office, each ticket group's Order URL can be found inside its edit dialogue. See attached.