Found it and verified regWrite() with Deye Cloud Read of System Work Mode-1 Page for 12k 3f model.
System Work Mode = REG 142
0 : Solar Sell
1: Zero Export to Load
2: Zero Export to Ct
@HonzaS Thanks!
Too bad REG 141 description is a total mess. I think it represents System Work mode.
I'd be interested switching between Zero Export to Load and Zero Export to CT by modbus command.