Should I use RouteParameter or UrlParameter for an Asp.NET web-api route?

I've seen both being used and so I wonder, do they do the same thing or different things? If it's the latter, what's the difference?

I tried answering it myself by having a look at the visual studio MVC 4 (rc) web api template, but sadly it uses both, so my confusion remains. Here's what the template contains:

public class RouteConfig
{
    public static void RegisterRoutes(RouteCollection routes)
    {
        routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

        routes.MapHttpRoute(
            name: "DefaultApi",
            routeTemplate: "api/{controller}/{id}",
            defaults: new { id = RouteParameter.Optional }
        );

        routes.MapRoute(
            name: "Default",
            url: "{controller}/{action}/{id}",
            defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
        );
    }
}