The Visa Bulletin, issued by the DOS, details the availability of immigrant visas. All beneficiaries, with some exceptions, of immigrant petitions cannot file an application for permanent residency unless their priority date is current and applications cannot be granted until an immigrant visa number is assigned to the application. Visa Bulletins are updated on a monthly basis. Preference categories have been set for both family and employment based applications.
Family Based Preference Categories
First: (F1) Unmarried Sons and Daughters of U.S. Citizens
Second: Spouses and Children, and Unmarried Sons and Daughters of Permanent Residents
A. (F2A) Spouses and Children of Permanent Residents
B. (F2B) Unmarried Sons and Daughters (21 years of age or older) of Permanent Residents
Third: (F3) Married Sons and Daughters of U.S. Citizens
Fourth: (F4) Brothers and Sisters of Adult U.S. Citizens
Employment Based Preference Categories
First: Priority Workers
Second: Members of the Professions Holding Advanced Degrees or Persons of Exceptional Ability
Third: Skilled Workers, Professionals, and Other Workers
Fourth: Certain Special Immigrants
Fifth: Employment Creation
Current and past visa bulletin can be found here.