According to John Higham in Strangers in the Land: Patterns of American Nativism: 1860-1925 (1963), the Act “proved in the long run the most important turning-point in American immigration policy” as it instituted numerical limits on immigration and a quota.
The objective of this act was to temporarily limit the numbers of immigrants to the United States by imposing quotas based on country of birth. Annual allowable quotas for each country of origin were calculated at 3 percent of the total number of foreign-born persons from that country recorded in the 1910 United States Census.
Exceptions to the quotas were made for government official and their families, aliens who were passing through the US or visiting as tourists or temporary workers, immigrants from countries in the Western hemisphere, and minor children of US citizens. The Quota Act also did not apply to countries with bilateral agreements with the US on immigration, or to countries in the Asiatic Barred Zone (as defined in the 1917 Immigration Act).