For future reference, in Windows the services file is %windir%\System32\drivers\etc\services. In Unix systems it is /etc/services. It is a mapping of service names to TCP or UDP port numbers.
The -S parameter, when used on a database broker, is what makes it a login broker. The broker binds to the specified TCP port and can accept incoming remote client connections and spawn servers to handle those clients. The -S parameter can be specified on the primary broker, and must be specified on any secondary broker. Each login broker in a database must have its own unique -S value.
When used as a client startup parameter, it causes the client to connect to the specified database via a remote server, by first connecting to the broker that is bound to the port specified by the value of the client's -S parameter.