MQL4参考 
标准常量，列举和架构 交易常数 订单属性
Order Properties
Operation type for the OrderSend() function. It can be any of the following values:
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OP_BUY
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0
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Buy operation
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OP_SELL
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1
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Sell operation
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OP_BUYLIMIT
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2
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Buy limit pending order
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OP_SELLLIMIT
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3
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Sell limit pending order
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OP_BUYSTOP
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4
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Buy stop pending order
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OP_SELLSTOP
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5
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Sell stop pending order
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