The UNique Ingredient Identifier(UNII) is a non-proprietary, free, unique, unambiguous, non-semantic, alphanumeric identifier linked to a substance's molecular structure and/or descriptive information by the Substance Registration System (SRS) of the Food and Drug Administration (FDA) and the United States Pharmacopeia (USP). A UNII is always ten characters long, and is randomly generated, so as not to contain any inherent information on time of entry or type of substance.
The SRS is used to generate permanent, unique identifiers for substances in regulated products, such as ingredients in drug and biologic products. The SRS uses molecular structure and descriptive information to define a substance and generate the UNII. The primary means for defining a substance is by its molecular structure as represented on a two-dimensional plane. When a molecular structure is not available (e.g., botanicals), the UNII is defined by descriptive information.
The procedures and management of the SRS is provided by the SRS Board which includes experts from both FDA and the United States Pharmacopoeia (USP).
Description taken from: https://en.wikipedia.org/wiki/Unique_Ingredient_Identifier