For a deeper look into our Elektron API, look into:
Overview | Quickstart | Documentation | Downloads | Tutorials | Articles
hEDD is a service name which provides data on your market data infrastructure. You should contact your market data admin to find out more about that service itself. If you are interested in using an API to get pricing data from it, then the best place to start is the real time SDK for Java or C++.
The data that is delivered on the real time feeds depends on the asset class as well as the listing venue. Use the Data Model Discovery and RIC search tool on the dev portal to learn more about this.