DDE is an acronym for Dynamic Data Exchange, which is a windows mechanism for inter process communication (IPC). Applications can communicate via DDE. Though DDE is a very old mechanism, nearly all windows applications are supporting those commands.
Normally you do not have the possibility to send DDE commands manually to an application (i.e. from a command line), but TeXnicCenter supports the special command line switch '/ddecmd' which sends the specified DDE commands to an already running instance of TeXnicCenter or if there is no running instance, start up a new instance and execute the DDE commands for that new instance.
The following DDE-commands are available:
If the specified file is a project file, the project will be opened, otherwise the file will be opened in the editor.