A DDE server uses a three-level hierarchy — service name (called "application name" in previous DDE documentation), topic name, and item name — to uniquely identify a unit of data the server can exchange during a conversation.
A service name is a string a server application responds to when a client attempts to establish a conversation with the server. A client must specify this service name to establish a conversation with the server. Although a server can respond to many service names, most servers respond to only one name.
A topic name is a string that identifies a logical data context. For servers that operate on file-based documents, topic names are typically filenames; for other servers, they are other application-specific strings. A client must specify a topic name along with a server's service name when it attempts to establish a conversation with a server.
An item name is a string that identifies a unit of data a server can pass to a client during a transaction. For example, an item name might identify an integer, a string, several paragraphs of text, or a bitmap.
The service, topic, and item names enable the client to establish a conversation with a server and to receive data from the server.