Sailfish2, a slave is a person who is the legal property of another and is forced to obey them.  The North Koreans who work in China and other countries are the legal property of North Korea and are forced to obey the NK government.  They do not travel on their own to work outside North Korea. They are sent there by the NK government and must remit most of their pay to the NK.  They work under prison-like conditions and to prevent them from defecting, their families are not permitted to go with them and instead are held hostage in North Korea. The conditions under which they work may be a lot better than in North Korea, but they nevertheless are slaves.