The Download Agent (DA) is a specialized binary file used by tools like the SP Flash Tool or UMT Dongle to communicate with the phone’s hardware. On the Vivo V9 (model 1723/1727), the DA file is essential for:
After a factory reset done via recovery, the Vivo V9 stores the previous Google account in the persistent partition. The setup wizard checks this partition. If it doesn't match, you are locked.