Does startService always create a new Service or is the Intent sent to the existing one?

I am developing an Android application where I start a Service via an Intent. The service opens a connection and reports its success to an Activity. Afterwars the user shall be able to start a data transmission via Button click. This is also done by the service.

Is it the best way to send another Intent to my service? And does a service always exist only once or is another instance created of it?