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The Immediate Relative class is regarded as a high priority classification and hence the US government has not imposed any annual number ceiling on this category and the family members of applicants can obtain permanent resident Green Card rapidly. Family members regarded as immediate relative includes following relations:
1. Spouse
2. Unmarried and dependent children under 21 years of age
3. Parents (if the US citizen is above 21 year of age)

The family preference class includes those members not included in the immediate relative class, i.e. the relatives in question are those family members of US citizens not defined in the immediate classification. In this category, the relations include following people
1. Unmarried sons or daughters aged over 21years
2. Married children of any age
3. Brothers and sisters (incase the U.S. citizen is aged over 21 years)
Congress has limited the number of relatives who may immigrate under these categories each year so there is usually a waiting period before an immigrant visa number becomes available.
The category of eligible family members includes relative classifications of both Immediate and Family Preference class.
There is a slight difference in requests placement process (petitioning) for people already in the US and those outside the US.
You can contact Vantage Immigration Consultancy services for further subtle elements on USA Green Card Visa. Vantage Immigration Consultancy services is a main Immigration Consultant houses master migration experts and caseworkers who resolve your inquiries and help you in a smooth movement process.