These are the codes that USCIS and the US State Department use to designate the legal basis for immigrant visas.  Each code identifies a separate legal basis for issuing an immigrant visa.  An applicant may qualify on more than one legal basis, but will generally need to choose a single legal basis for an immigrant visa.

CodeTypeDescriptionA11Immigrant VisaUnmarried Amerasian son or daughter of a US citizen born in Cambodia, Korea, Laos, Thailand, or Vietnam.  INA 203(a)(1) and 204(g).A12Immigrant VisaChild of an alien classified as A11 or A16.  INA 203(d), 204(g).A16AdjustmentUnmarried Amerasian son or daughter of a US citizen born in Cambodia, Korea, Laos, Thailand, or Vietnam. INA 203(a)(1) and 204(g)A17AdjustmentChild of an alien classified as A11 or A16. INA 203(d), 204(g).A31Immigrant VisaMarried Amerasian son or daughter of a US citizen born in Cambodia, Korea, Laos, Thailand, or Vietnam. INA 203(d), 204(g).A32Immigrant VisaSpouse of an alien classified as A-31 or A36.  INA 203(d), 204(g).A33Immigrant VisaChild of an alien classified as A31 or A36.  INA 203(d), 204(g).   C20AdjustmentChild of an alien classified as C24 (unmarried son or daughter stepchild of permanent resident - immigrant visa) or C29 (unmarried son or daughter stepchild of permanent resident - adjustment).  INA 203(d), 216.C21Immigrant Visa or AdjustmentSpouse of a lawful permanent resident.  INA 203(d), 216.C22Immigrant Visa or AdjustmentStepchild (under age 21) of a lawful permanent resident.  INA 203(d), 216.C23  C24Immigrant VisaUnmarried son or daughter (21 or older) who is a stepchild of a lawful permanent resident.  INA 203(d), 216.