There are many sources of law governing immigration. First among these is the United States Constitution.
Statutes usually provide broad authority and apply to general situations.
Agencies must take the general provisions of the statute and apply them to specific, detailed situations. The INS regulations serve this purpose. For example, both the Department of Labor and Department of State are involved in the administration of the immigration laws. One source of legal interpretations of immigration laws and regulations are administrative decisions. The Board of Immigration Appeals is a separate agency within the Department of Justice that reviews immigration cases and issues appellate administrative decisions that are binding on the INS nationwide.