Short answer:

An object is attached to a scene, an item is attached to an inventory.

Long answer:

An object is associated with an implicit action within a scene, whereas an item is associated with an inventory. An object appears in the noun list of the parser when the associated implicit action comes into scope (you have reached the corresponding scene/event in the narrative in which the action can be applied). An item appears in the noun list of the parser when the item is in the active inventory.

An example would be "open door with key"; 'door' would be declared as an object and 'key' as an item. The parser would list 'door' as an option when the implicit action is in scope, however it will not list key unless the item 'key' is in the inventory. Thus the reader cannot perform the action 'open door with key' until they have acquired the key, however they do know that something can be done with 'door'.