In most countries it is illegal to possess narcotic drugs. The same applies in the Netherlands, where the rules are laid down in the Opium Act and associated legislation. The appendices to the Opium Act state which substances are covered.
Category I drugs are considered to be hard drugs, and category II drugs soft drugs. Sentences are severe, and longer sentences are given for hard drugs offences. The relevant legislation is available on this website;
Although the Office for Medicinal Cannabis Research (OMC) is a Dutch government agency, its activities are not limited to Dutch territory or to the situation in the Netherlands. The OMC is obliged to observe international United Nations legislation, not just the Dutch Opium Act. The UN agency responsible for the substances referred to in the Opium Act is the International Narcotics Control Board (INCB). The OMC provides the INCB with regular updates on the situation in the Netherlands. You can read the letters here: