When you connect to our of our Servers the real-time streaming data is published under one or more services i.e. sources of data e.g. our Real-time feed is published under ELEKTRON_DD
However, if you are connecting to an internal server at your organisation, it may have several other services including 3rd party sources, internal sources or even delayed or conflated versions of ELEKTRON_DD
To subscribe to data from a particular source you need to know the specific service name for that source.
DIRECT_FEED is just a dummy service name used by most of our examples - as you can run a dummy Provider example which will publish under the DIRECT_FEED name and direct connnect the consumer to the dummy provider to consume the dummy data.
The data-services on the market data server can have different properties like real-time, or delayed or conflated data. Sometimes, administrator also partition the data by asset classes so that equity instruments are available on a different service vs options data. So serviceName is just what is defined in your MDS. Your admin can advise which service name to use in your application.