Steps to reproduce:

1. https://www.zabbix.com/documentation/3.4/ru/manual/api/reference/item/get
2. Look at selectItemDiscovery to see how it's work.. and.. how it's work?
3. Right answer is selectItemDiscovery="extend" (in json as example {"hostids": 10923, "selectItemDiscovery": "extend"})
4. Not enough example for it

Result:
See screenshot...
Expected:
Full example or manual for it