These classes, present in earlier versions of the cgi module, are still supported for backward compatibility. New applications should use the FieldStorage class.
SvFormContentDict stores single value form content as dictionary; it assumes each field name occurs in the form only once.
FormContentDict stores multiple value form content as a dictionary (the form items are lists of values). Useful if your form contains multiple fields with the same name.
Other classes (FormContent, InterpFormContentDict) are present for backwards compatibility with really old applications only. If you still use these and would be inconvenienced when they disappeared from a next version of this module, drop me a note.