What is the difference between AI (01) and AI (02)?

The Application identifiers (AIs) (01) and (02) are used to indicate Global Trade Item Numbers (GTINs).

Application Identifier (01)

AI (01) indicates the GTIN (barcode number) of the actual item being labelled e.g., a consumer unit or a logistics unit such as an inner, an outer case or a pallet.
AI (01) is used on a logistics unit where the unit has its own unique GTIN.
This GTIN is distinct from the GTIN of the product contained inside it.

Application Identifier (02) + (37)

AI (02) is used to indicatethe GTIN (barcode number) of the item contained within the logistics unit IF the logistics unit itself does not have its own GTIN. 
This is often used for items identified by the same GTIN and that do not have a corresponding logistics unit GTIN
AI (02) MUST always be used in conjunction with AI (37) which declares the quantity of the items within the logistics unit.

Example 1:

A pallet containing 120 cases of a product identified with GTIN 5391234000016, is assigned a pallet GTIN of 5391234000023. The encoded identifiers will be:

Pallet: (01)5391234000023

Case: (01)5391234000016