Open In Worksheet and Open Data
The File | Open in Worksheet command or the button opens a data file in a worksheet window. In the Open Data dialog, the file types shown are limited to Common Data Files (*...). Compare this with the File | Open command and its Open dialog which opens maps, not worksheets, regardless of the file type. Hold down the CTRL key while clicking on files to select multiple files, or hold down the SHIFT key to select adjacent files. Then click Open and the data files will open in separate worksheet windows.
When a worksheet is already open, you can click the Data | Edit | Open Data command to open a data file. The Open dialog that opens for Data | Edit | Open Data is same as that for the File | Open in Worksheet dialog.