The second engine of a VPN client, an alternative to Xray: the same protocols, a different implementation. Two engines let the client switch when one has problems.
In OshaVPN both engines are connected through a thin layer, so an engine can be swapped without rewriting the app. On Windows 7 only sing-box works.
In OshaVPN sing-box was the first engine; Xray is now the main one. The UI, subscription and routing talk to the engine through a narrow engine-api layer, so the engine is switched by a setting, with no reinstall. sing-box is kept for comparison, for Windows 7 and for the Hysteria2 and TUIC protocols. On Android both engines are built into one library; on Windows they run as separate processes.
If all users are on modern systems and the main engine is enough, a second engine is extra code. We keep it for old Windows and as a fallback path.
Terms explained — now let's get to work: tell us about the task and we'll turn it into a clear work plan.