Application Identifier 01 is used to identify a trade item with a GTIN: GTIN-13, GTIN-14 or GTIN-12.
AI (01) is used when identifying a standard unit, such as a case or a pallet containing the same items each time the case or pallet is ordered.
When identifying with GTIN-13 or GTIN-12, leading zeros are added so that all 14 digits in the data format are completed.
If the GTIN is prefixed with the digit 9, this indicates that the product is of variable measure. In such cases, it must be used together with either AI 30, 3nnn, or 8001.
If you’d like, I can align the terminology precisely with GS1 General Specifications wording.