主题：条件累加代码，求解说

For i As Integer = Tables("表A").count-1 To 0 Step -1  
            Dim r As Row = Tables("表A").Rows(i)
            If  i = Tables("表A").count-1
                r("累计成本") = r("系数") * r("单价")
            Else
                If r("加工级别") <> Tables("表A").Rows(i+1)("加工级别")
                    r("累计成本") = DataTables("表A").Compute("Sum(累计成本)", "加工级别 =" & _
                    Tables("表A").Rows(i+1)("加工级别") & " ") + r("单价")
                Else
                    r("累计成本") = r("系数") * r("单价")
                End If
           End If
Next

r("累计成本") = DataTables("表A").Compute("Sum(累计成本)", "加工级别 =" & _  

Tables("表A").Rows(i+1)("加工级别") & " ") + r("单价")   这些不太懂   _是什么？