参考：http://www.hooyou.com/familybased/index.html

就是说不同亲属种类，有不同的配额限制，透过每个月的排期表就可以估计自己的申请大概多少年可以被批准。
Each Preference is given a numerical quota per year to limit the number of immigrants admitted into the United States

Each Preference is given a numerical quota per year to limit the number of immigrants admitted into the United States. - See more at: http://www.hooyou.com/familybased/index.html#sthash.lpv8xiDa.dpuf