Pijiang: I got my Green Card. What's White Paper for ?

From what I have heart that "White Paper" for a GC holder, who assign to foreign country to work and have to keep his or her GC status, apply for Green Card landing in the U.S. beyond 180 days each year. It could help GC holder to land the U.S. every two years. Could you help me to understand what the White Paper is? Some people said it's just like Advance Parole. Thanks so much.