Permanent Employment-Based Immigration and the Per-Country Ceiling [Updated December 21, 2018] [open pdf - 1MB]
"Four major principles currently underlie U.S. policy for admitting lawful permanent residents (LPRs): reunifying families, admitting individuals with needed skills, providing humanitarian assistance, and diversifying immigrant flows by country of origin. These principles are expressed in the Immigration and Nationality Act (INA), which contains a pathway for acquiring LPR status for each principle. Family reunification occurs primarily through family-sponsored immigration. Admitting individuals with needed skills occurs through employment-based immigration. Humanitarian assistance occurs primarily through the U.S. refugee and asylum programs. Origin-country diversity occurs primarily through the Diversity Immigrant Visa. This report focuses on the second LPR pathway noted above, employment-based immigration. Employment-based immigrants enter the United States through one of five preference categories, each with its own eligibility requirements and numerical limitations and, in some cases, different application processes."
CRS Report for Congress, R45447
Congressional Research Service: https://crsreports.congress.gov/