There are several services defined in AOO API that implement a single interface and are defined in IDL using the old-style

service Something {
interface XSomething
};

This old-style service definition has several drawbacks 

* in order to create an instance of the service a string with the fully qualified service name has to be used, which is rather error prone

* in strong typed languages, the instantiated object has to be queried for the interface it implements

* ...

These service definitions can be converted to new-style service definitions avoiding the problems mentioned above:

service Something : XSomething;

or spelling the default service constructor explicitly:

service Something : XSomething { create(); };

The explicit service constructor has the advantage of being more clear, stating that this service is a new style-service, differentiating it from other services that are not supposed to be instantiated, like abstract services (services that only define a possible implementation but are not instantiated by name, for example css.frame.PopupMenuController/StatusbarController/ToolbarController), services that describe only properties, etc.