Steps to reproduce:
Open: https://www.zabbix.com/documentation/4.0/manual/api/reference/item/object

Result:

(readonly) Origin of the item. 

Possible values: 
0 - a plain item; 
4 - a discovered item.

Expected:

• 0 Normal item
• 1 Discovery rule
• 2 Item prototype
• 4 Auto-created item