Read this and bottom link and share with us what u understand the USCIS policy.
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Overview of Immigration Process
A legal immigrant (or “lawful permanent resident”) is a foreign national who has been granted the privilege of living and working permanently in the United States. There is a three-step process for your child or son or daughter to become a legal immigrant.
You must obtain USCIS approval of an immigrant visa petition that you file for your child, son or daughter.
The State Department must then give your son or daughter an immigrant visa number, even if he or she is already in the United States. If you are a U.S. citizen and the child is both under 21 years of age and unmarried, a visa number is not required.
If your child or son or daughter is outside the United States, he or she will be notified to go to the local U.S. consulate to complete the processing for an immigrant visa when one becomes available. If your child or son or daughter is legally in the U.S. when an immigrant visa number becomes available (or if one is not required), he or she may apply to adjust status to that of a lawful permanent resident using the Form I-485.
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for more detail read this and share with us what u understand
http://uscis.gov/graphics/faqsgen.htm#derivative
Good Luck