Per country levels for family-sponsored and employment-based immigrants
Under the per-country cap set in the Immigration Act of 1990, no country can receive more than 7 percent of the total number of employment-based and family-sponsored preference visas in a given year.
There are about 200 countries in the world. Immigrants can come from practically any of those countries. If an equal number of the immigrants came from 14 countries, they would all be at roughly 7%. Of course, nearby countries, Mexico in particular, are more likely have citizens wishing to immigrate here that some country on the other side of the globe.
The seven percent cap strikes me as arbitrary. Why should a would-be visa-holder from Latin America be dependent up the percent of immigrants coming from other countries? More logically whether or person is allowed to immigrate into America should be decided individually regardless of their country of origin.