When it comes to becoming a United States citizen, family members could also end up sponsoring the person. However, when it comes to immediate family or even an immediate relative, there are certain definitions that come into play. The immediate relatives for a US citizen would be defined as a spouse, or unmarried children who happen to be under the age of 21. It could also be parents for the person. Immediate relatives will always have a Visa number available for the person.
This has been basically done in order to promote the family unity and as has been required by the immigration law in which the United States citizens have been known to petition for certain qualified family members in order to come and live with them permanently in the United States. The best thing about this policy is that the immediate relatives have always got a priority over all the others, and they do not have to wait in line in order to get a Visa number. It would be directly available to them because there are unlimited numbers of such visas available in these categories itself.
For a green card, there are certain procedures that need to be followed. Certain people are also eligible in order to apply for the Green card while they are inside the United States of America. It is an immediate relative which needs to be established as a relationship that can enable you to apply for this particular process. After the fulfillment of this application, you would become a permanent resident of the United States.
If you happen to reside outside the United States, and you have an immediate relative as a US citizen, then you need to get permanent residency through the consular processing. The consular processing is where the Department of the State will be able to issue a visa that will be approved only when the petition is filed. However, you would need to travel to at least one entry port in the United States before you would be applicable for this Visa.
It is also the job of the Department of State to inform you in case you would be eligible to apply for an immigrant visa. If you do not apply for the immigrant Visa within the year, then your petition will be terminated. However, there are certain things for you to note. If the immediate relative is going to get married under the age of 21, then he or she will not be classified as an immediate relative, and it would then be classified as a third preference category. That means that the naturalization process which normally accompanies the unlimited Visa preference would no longer be available to you. Therefore, you would need to notify the changes immediately otherwise you end up being guilty in the immigration process.
Further Reading: https://travel.state.gov/content/visas/en/immigrate/family/family-preference.html