From the Document: "Congress has enacted a series of legislative provisions since 2006 to enable certain Iraqi and Afghan nationals to become U.S. lawful permanent residents (LPRs). These provisions make certain Iraqis and Afghans who worked as translators or interpreters, or who were employed by, or on behalf of, the U.S. government in Iraq or Afghanistan, eligible for 'special immigrant' visas (SIVs). Special immigrants comprise a category of permanent employment-based admissions under the Immigration and Nationality Act (INA). While the special immigrant category is unique, it does bear some similarities to other admission categories that are authorized by other sections of the INA, including refugees and Amerasian children."
CRS Report for Congress, R43725
Congressional Research Service: https://crsreports.congress.gov/