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(a) Preference Allocation for Family-Sponsored Immigrants. - Aliens subject to the worldwide level specified in section 201(c) for family-sponsored immigrants shall be allotted visas as follows:
(FB1) Unmarried sons and daughters of citizens. - Qualified immigrants who are the unmarried sons or daughters of citizens of the United States shall be allocated visas in a number not to exceed 23,400, plus any visas not required for FB4.
(FB2) Spouses and unmarried sons and unmarried daughters of permanent resident aliens. - Qualified immigrants -
(A) who are the spouses or children of an alien lawfully admitted for permanent residence, or
(B) who are the unmarried sons or unmarried daughters (but are not the children) of an alien lawfully admitted for permanent residence, shall be allocated visas in a number not to exceed 114,200, plus the number (if any) by which such worldwide level exceeds 226,000, plus any visas not required for FB1; except that not less than 77 percent of such visa numbers shall be allocated to aliens described in subparagraph (A).
(FB3) Married sons and married daughters of citizens. - Qualified immigrants who are the married sons or married daughters of citizens of the United States shall be allocated visas in a number not to exceed 23,400, plus any visas not required for FB1 and FB2.
(FB4) Brothers and sisters of citizens. - Qualified immigrants who are the brothers or sisters of citizens of the United States, if such citizens are at least 21 years of age, shall be allocated visas in a number not to exceed 65,000, plus any visas not required for FB1 through FB3.
This describes a circular system, whereby spare visas can fall down the FB classes and then back up to the top if necessary:
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(b) Preference Allocation for Employment-Based Immigrants. - Aliens subject to the worldwide level specified in section 201(d) for employment-based immigrants in a fiscal year shall be allotted visas as follows:
(EB1) Priority workers. - Visas shall first be made available in a number not to exceed 28.6 percent of such worldwide level, plus any visas not required for EB4 and EB5, to qualified immigrants who are aliens described in any of the following subparagraphs (A) through (C):
(EB2) Aliens who are members of the professions holding advanced degrees or aliens of exceptional ability. -
(A) In general. - Visas shall be made available, in a number not to exceed 28.6 percent of such worldwide level, plus any visas not required for EB1, to qualified immigrants who are members of the professions holding advanced degrees or their equivalent or who because of their exceptional ability in the sciences, arts, or business, will substantially benefit prospectively the national economy, cultural or educational interests, or welfare of the United States, an d whose services in the sciences, arts, professions, or business are sought by an employer in the United States.
(EB3) Skilled workers, professionals, and other workers.-
(A) In general. - Visas shall be made available, in a number not to exceed 28.6 percent of such worldwide level, plus any visas not required for EB1 and EB2, to the following classes of aliens who are not described in paragraph (2):
(EB4) Certain special immigrants. - Visas shall be made available, in a number not to exceed 7.1 percent of such worldwide level, to qualified special immigrants described in section 101(a)(27) (other than those described in subparagraph (A) or (B) thereof), of which not more than 5,000 may be made available in any fiscal year to special immigrants described in subclause (II) or (III) of section 101(a)(27)(C)(ii) , 2/ and not more than 100 may be made available in any fiscal year to special immigrants, excluding spouses and children, who are described in section 101(a)(27)(M) .
(EB5) Employment creation. -
(A) In general. - Visas shall be made available, in a number not to exceed 7.1 percent of such worldwide level, to qualified immigrants seeking to enter the United States for the purpose of engaging in a new commercial enterprise (including a limited partnership)--
This describes a system where unused EB4 & EB5 visas can fall up to EB1. Unused visas can fall down from EB1 to EB2 or EB3 and unused visas in EB2 can fall down to EB3.