ISO 11407:2004 specifies the interchangeability requirements -- the various elements and dimensions -- for the mechanical coupling of towing vehicles whose coupling is mounted forward and below to centre-axle trailers. It is applicable to road trains for the commercial transport of low-density, high-volume cargo of the greatest possible variety. It is also applicable to corresponding equipment in cases where the same vehicle makes up part of a road train having more than one trailer. It is not applicable to dedicated or special combinations such as coupling systems with an extensible drawbar; nor does it specify limitations on maximum gross mass and overall dimensions, which are generally laid down by legislative requirements.