Congress created three groups in order of preference for granting green cards through employment.  These three employment-based preference groups, usually requiring an offer of employment in the United States, are:

• the first employment-based preference (about 40,000 annual visas) for "priority workers":

(1) managers and executives subject to international transfer to the United States (no labor certification required);

(2) outstanding professors and researchers with universities or private employers that have established research departments (no labor certification required);

(3) aliens of "extraordinary ability" in the sciences, arts, education, business, and athletics (no labor certification required)(no offer of employment required).

• the second employment-based preference (about 40,000 annual visas plus visas not used in the first preference):

(1) aliens of "exceptional ability" in the sciences, arts, or business;

(2) advanced-degree professionals.

A PERM labor certification is required, and a job offer is also required, unless it is waived in the national interest; if the job offer is waived by the USCIS in the national interest, the alien is not subject to the labor certification requirement normally applicable to second preference aliens. National interest waivers have become an important feature of second preference cases.

• the third employment-based preference (about 40,000 annual visas plus visas not used in the first and second preferences):

(1) professionals with bachelor's degrees not qualifying in the second preference;

(2) skilled workers (filling positions requiring at least two years of training and experience);

(3) unskilled workers.

A PERM labor certification and an offer of employment are required. Only 10,000 visas of the annual allotment may be assigned to unskilled workers, effectively creating a separate subpreference for those workers.