The inventory item data type refers to the items that are listed in the game's OBJECT file.
Inventory items are referenced in logics by an unsigned eight bit number that corresponds to the index of the item in the OBJECT file.
The first inventory item (index 0) in all known Sierra games is the single character '?'. Other item indices that aren't used by the game are also set to this single question mark character. Setting the first item to a single question mark does not appear to be required. Most fan made games continue using the question mark for item 0 and any unused/undefined items.