Bibliographic styles may require the book edition number to be a cardinal or an ordinal number. If your edition number is stored in the database as a cardinal number, then it will be formatted as an ordinal number if required by the bibliographic style. If your edition number is stored as anything other than a cardinal number it will be used unchanged. The conversion is English - i.e. '3' => '3rd'. This works all the way up to infinity-1 ;-)