DDE uses the three-level hierarchy-application, topic, and item-to uniquely identify a unit of data. An application is the name of the server from which the data is desired. A topic is a logical data context. For applications that operate on file-based documents, topics are usually file names; for other applications, they are other application-specific strings. An item is a data object that can be passed in a DDE transaction. For example, an item might be a single integer, a string, several paragraphs of text, or a bit map. In the collector and spreadsheet model described in the previous section, the application name is collector, the topic name is STOCKS, and the item name is ABCD.