Flat files are ASCII text files that contain no formatting and have no column headers. They are convenient to use with statistical processing programs such as SPSS or SAS.
Text files are ASCII text files with a minimum of formatting and can be read like flat files in text viewers (e.g., Notepad).
The general documentation file includes the record layout and the survey form.
More recent files (or recently revised files) use version letters, "a" being the first release, "b", "c", etc. representing revisions of the file.