For standard configurations, the second and third columns, if present, are used on the fax header as a To: address to provide better delivery to the named recipient. The text from the two columns is separated by a '/' character and may contain any text so, for example, you could put a company name and general job title if you are not able to fax to a specific person. The maximum number of characters available is generally enough to print both a long full name and a long company name but varies depending on the text because the font used is a variable width font rather than a fixed width font.
Columns beyond 3 are stored on the system and can be used for customized drop in text. Our default labels for the next 5 are Addr1, Addr2, City, State, Zip. There are an additional 14 columns beyond these that are generically labeled field7 through field20.
If your list data has columns listed in a different order, you can check the change column order checkbox and you will get a chance to re-order your columns during the import process.
Example with names:
555-555-1212<tab>John Smith<tab>Widgets R Us
555-555-1214<tab>Office Manager<tab>Flowers To Go