Visa Numbers
This explains visa number usage, spillover of visa numbers into other preference categories, and the visa number backlog.
Preference Categories
The primary reasons for entering the immigration process in the U.S. are family and employment-based. To manage this process, family and employment-based immigration is broken into preference categories. Congress sets limits on how many individuals may immigrate to the U.S. each year (i.e. visa numbers) and also sets limits on how many individuals may immigrate within each preference category.