What do the F1, F2A, F2B, F3, and F4 family preference categories mean?

The family preference system uses categories to set the order in which limited visas are distributed. First preference (F1) covers unmarried sons and daughters age 21 or older of U.S. citizens. Second preference is split into F2A for spouses and unmarried children under 21 of lawful permanent residents, and F2B for unmarried sons and daughters 21 or older of lawful permanent residents.

Third preference (F3) covers married sons and daughters of U.S. citizens, and fourth preference (F4) covers brothers and sisters of U.S. citizens who are at least 21. Each category has its own annual allotment and waiting line, so two relatives with the same priority date can wait very different lengths of time depending on category and country.