functionScore 里的scoreMode与boostMode区别有什么区别

{

  "function_score" : {

    "query" : {

      "bool" : {

        "must" : [ {

          "term" : {

            "isCollect" : 0

          }

        } ]

      }

    },

    "functions" : [ {

      "field_value_factor" : {

        "field" : "initScore",

        "missing" : 0.0

      }

    }, {

      "exp" : {

        "updateTime" : {

          "origin" : "now",

          "scale" : "1d",

          "decay" : 0.8,

          "offset" : "1d"

        }

      }

    } ],

    "score_mode" : "sum",

    "boost_mode" : "sum"

  }

}

如上这段代码,最终参与 exp函数的_score是原本的_score还是 _score+

initScore 求指导，另外有人能细说一下score_mode和boost_mode的区别么