For heavy duty vehicles, as a general rule of thumb:
Most vehicles manufactured in 2007 or later will have the 9-pin diagnostic port
Vehicles manufactured from 2002-2006 may have either the 9-pin or 6-pin diagnostic ports
Vehicles built from 1996-2001 generally have the 6-pin diagnostic port
If you have a model year 2013 or newer Mack or Volvo truck, you should not order a 9-pin or 6-pin cable. You should order the cables specifically designed for model year 2013 or newer Mack or Volvo trucks.