what's the field "exchange_rate" mean in order transaction api

[chenshi]

hi,excuess me

when i call the order transaction i see the field  "exchange_rate" under the receipt obj 

{

    "transactions": [

        {

         ….

          "gateway": "paypal",

          ….

            "receipt": {

             ….                                       

                "exchange_rate": "0.75223319228961",

             ‘’‘’‘

            },

        }

    ]

}

i don't know what's the field mean as this order was paid by paypal ; but  in the paypal report  fields doc 

https://www.paypalobjects.com/webstatic/en_US/developer/docs/pdf/PP_LRD_Gen_TransactionDetailReport....

i also can't find the filed exchange_rate,,,,,,,tks