It's exactly what it sounds like. The intent of this requirement is to have the home country benefit from the Exchange Visitor's experience in the United States. Exchange Visitors come to this country for a specific objective such as a program of study or a research project. The requirement is intended to prevent a participant who is subject from staying longer than necessary for the objective, and to ensure that he or she will spend at least two years in the home country before coming back to the United States for a long-term stay.
H includes temporary workers, trainees, and their dependents.
L includes intracompany transferees and their dependents. The A classification includes your home government's diplomats and representatives to the United States government, and their dependents. The G classification includes your government's representatives to international organizations, such as the United Nations, and their dependents. Even though these endorsements are not final, INS usually accepts indications that the Exchange Visitor is subject. If you are unsure whether or not you are subject , consult the OISS staff or your J-1 Responsible Officer.