The (often named MTK_All_In_One_DA_5.1648.bin or similar) is a custom, patched, or aggregated Download Agent that bypasses many of the security restrictions imposed by official DAs.
Newer or high-security devices (like some Samsung or Vivo models) often require a custom DA file specifically for that chipset or brand. Without the correct custom DA, users often encounter errors or the device simply fails to connect. How to Use the DA File in SP Flash Tool Importance of DA Files for MTK Devices | PDF - Scribd mtk all in one da file