>>Nope - an SDF file is nothing more or less than a text file with a fixed number of characters per line.
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>Appreciate the clarification Ed. One more questions - the icon for a text file is different from that of a file without an extension. Why does that happen? Implication?

The file extension is used to deterine the file type; for each file extension, there is an entry in the registry under HKEY_CLASSES_ROOT specifying a relation to a ProgID for that file extension, and one of the items stored beneath the ProgID is the Default Icon for that file type.

The implication is that the file extension specifies the registry-associated behavior for the file - what happens if you click on the file or right-click on the file in Explorer is determined by the Shell entries under the ProgID, including the default behavior (normally "OPEN") and the specific entries shown on the context-sensitive menu.