It has been law since 2005 that all horses, ponies and donkeys have a passport which identifies the animal and its owner.
Horse meat is eaten throughout Europe. Some medicines used by vets cannot enter the human food chain. By signing the appropriate part of your passport you can prevent your horse entering the food chain and thus give permission to the vet to use any drug without any concern that the drugs will ever enter the food chain.
It is now the law that all horses born after 1st July 2009 or any horse having its first passport will need to be microchipped.