The default application is the one which opens when you open a file.
Mac OS X Server matches applications with files they can open automatically. You can't influence the available default applications for files in any way other than by installing or removing applications.
To change a file's default application, follow these steps:
1. In the Workspace Manager, select the file by clicking on it.
2. Open the Tools menu and choose Inspector.
3. Choose Tools from the Inspector pop-up menu.
4. Select an application's icon from the center of the window.
5. Click Set Default.