The Dutch Harness Horse is a type of horse

The type is the same in The Netherlands as it is in the United States.
Breeding for a type means that horses, including Dutch Harness Horses, are not by definition a pure or closed breed.
Pure breeding is not a goal. Breeding stock can be used from suitable sources, not just with parents who are already registered as Dutch Harness Horses.

A purebred is a restricted breed and no horses from other registries may be used for breeding regardless of whether they fit the desired type.

ADHHA

ADHHA is a registry with breeding goals to achieve the same type of horse the Dutch KWPN registers as Dutch Harness Horses. Pureness of blood is not a criteria.

Breeding high steppers
ADHHA's only breeding goal is to breed the type of the high stepping show horse.
Fitting the type is the criteria for KWPN registration, just as it is for ADHHA registration.

 KWPN

Since 2006 the KWPN is divided up into the following 4 discipline based types (source KWPN website):

• dressage
• jumping
• Gelder Horse
• Harness Horse

Four breeding goals
Each discipline has their own type, their own breeding goal and their own ideal use. The highest level in the dressage discipline, the highest level in the jumping discipline and the highest level in the show ring for high steppers.
Gelder horses have no specific goal. A Gelder Horse is bred to be versatile, basic horse.

Gelder Horse Type

The Gelder Horse is not a show horse type. A Gelder Horse is bred to be a versatile, basic horse. Some are suited for combined driving and dressage.
The Gelder Horse breeding goals for motion and type do not fit the ADHHA or KWPN Harness Horse breeding goal.

Harness Horse Type

The Dutch Harness Horse is specifically bred to perform in harness on the highest level in the show ring. Show Horse type and motion is defined in the breeding goal for achieving this specific type of suitability.
Today, a Dutch Harness Horse which doesn't achieve the breeding goal might fit the Gelders Horse goal, but not vice-versa. Horses with Harness Horse blood may also be found in dressage and combined driving competition. Some are successful at the highest levels of these disciplines.