Certain categories of visas are subject to yearly numerical limitations (Quotas). When more approved applications for visas are received, than are available visas, waiting lists develop.

Approved applications for each type of visa are place in line by date and are approved when an opening comes up. The Department of State issues a Monthly Bulletin listing the dates that each category is up to. Some visa categories tend to stay current (C), that is, with no waiting list. Some have cut-off dates which means that only applicants with dates (visa numbers) prior to that date can get a visa for that month. Some categories for some countries may be unavailable (U).

Currently the Quota system separates immigrant visas into family and employment based categories. Certain immigrants are not subject to numerical limitations. Additionally there are visas per countries limits set up along with the category limitations.