What is an immigration petition, and when is it necessary to file one?

Petition-based work visa categories require a petition to be filed with U.S. Citizenship and Immigration Services (USCIS) before the foreign national can apply for the visa stamp abroad at the U.S. Consulate or Embassy in their home country. Petition-based categories include H-1B, L-1 Individual (where the company does not hold a Blanket Approval for L-1s), and O-1. Consular-based categories do not require this step and the foreign national can complete the entire process through their local U.S. Consulate or Embassy. Consular-based categories include L-1 Blanket, TN for Canadians and Mexicans, E-3 for Australians, H-1B1 for Chileans and Singaporeans, E-1, and E-2.