The term Unique Item Identifier (UII) refers to the string of digits that compose the UID/IUID (Item Unique Identifier) as opposed to the 2D barcode symbol below. An example of a UII: D1HLD9PIZZAOVEN02882928. It is composed of a leading IAC ('D') which indicates that the next part is a CAGE ('1HLD9') and the remainder a part number and serial number, or just a sequence number.
IUID Data Matrix
The UID/IUID is a Data-Matrix symbol that includes additional characters allowing machines to understand them, including their exact components (e.g. CAGE, Part Number and Serial Number). For this reason they are also referred to a Machine Readable Interface (MRI). See IUID - Item Unique Identification for more information.
While MRIs / UIDs are very useful in situations where scanners are available they cannot be other very common situations, like Mil-Std container labels, WAWF transactions, spreadsheets and emails.