We seem to have no documentation on the available routing loader types that you configure when loading a resource in app/config/routing.yml. Nor do we document how Symfony determines the loader when no type is used and what the benefit of the type setting is. I've seem a couple of people asking for such docs the last weeks.

I think this can be added to book/routing directly in the "Including External Routing Resources" section.

Available loaders: yaml, php, xml, annotation (if SensioFrameworkExtraBundle is registered)

How loaders are choosen
When no type is configured, it's based on the extension. The first matching loader will be used. When a type is configured, both extension and type are taken into account. For instance, a file custom_routing.yml with type yaml will be loaded by the yaml loader, while a custom_routing.php with type yaml will not be loaded, as no loader matches both the .php extension and the yaml type.

The benefit of the type setting
Types allow you to use different loaders for the same extension, for instance a .php extension can be loaded by both the PHP and the annotation loader. Using the type annotation will make sure it's loaded by the annotation loader.

/refs symfony/symfony#13623