I don't want big slippage happen while opening or closing orders.
However, the slippage is still often much higher as over "10" although I set "slippage=3" already.
Now that the slippage in "OrderSend()" and "OrderClose()" don't work at all, why MQ team added it as the function parameters?
How can I exactly prevent big slippage happen with my code?
They didn't add it when it doesn't work; it was in there before ECNs when it did. Removing it would break existing code.
There is nothing that can be done in your code; only in the broker's server.