Preference relatives are certain family members of a U.S. citizen who are seeking to immigrate to the United States based on their relationship to the U.S. citizen. There are yearly limitations on family preference immigrants and the relatives in this category must often wait several years before an immigrant visa becomes available. The available visas are issued in the order in which the petitions were filed (the priority date).
Bureau of Consular Affairs, Family Based Immigrants, travel.state.gov/visa/immigrants/types/types_1306.html
USICS, Family of U.S. Citzens, www.uscis.gov/portal/site/uscis/menuitem.eb1d4c2a3e5b9ac89243c6a7543f6d1a/