This pane is used to display any type of document in that window pane, instead of displaying the document in its current location.
After opening the Document Dock pane, you can move any active document into it manually by selecting Window>Send To Other Dock.
You can also specify that certain types of files (those with a particular file extension) will open automatically in the Document Dock pane. To do this, select Tools>Options, and in the Options dialog, enter the appropriate file extensions in this field, separating each one with a semi-colon (e.g., xml;text;asp).