蓝总：以下是论坛<驾驶员理论考试模拟系统》的关于如何组组题的代码，就这个实现看的云里雾里，能不能将黄色部分关键代码解释下

Tables("试题库").Redraw = False

Tables("试题库").Filter = ""

Tables("试题库").Sort = ""

For i As integer = 0 to Tables("试题库").rows.count -1

    Tables("试题库").Rows(i)("随机值") = Rand.Next(10000)

Next \'排随机值

For Each dr As DataRow in DataTables("试题库").DataRows

    dr("信息") = Nothing

    dr("序号") = Nothing

Next \'清空指定列的数据

Dim fls As List(Of String) = DataTables("试题库").GetUniqueValues("","分类")

For Each fl As String In fls

    \'获得该分类的全部行

    Dim drs As List(Of DataRow) = DataTables("试题库").Select("[分类] = \'" & fl & "\'", "随机值")

    For n As integer = 0 To drs.Count - 1 \'遍历所有行

        drs(n)("组题") = n + 1 \'编号排序

    Next

Next

Dim Str As String

For Each fl As String In fls

    Dim dr As DataRow = DataTables("组题方式").Find("[试题] = \'" & fl & "\'")

    \'组合字符串

    str = str & " or [分类] = \'" & fl & "\' And [组题] <= " & dr(Vars("车型"))*100

Next \'筛选

Tables("试题库").Filter = str.SubString(4)

Tables("试题库").Sort = "随机值,题型"

For i As integer = 0 to Tables("试题库").rows.count -1

    Tables("试题库").Rows(i)("序号") = i + 1

Next \'对序号我按筛选行的顺序赋值

Tables("试题库").Redraw = True

Dim Str As String

For Each fl As String In fls

    Dim dr As DataRow = DataTables("组题方式").Find("[试题] = \'" & fl & "\'")

    \'组合字符串

    str = str & " or [分类] = \'" & fl & "\' And [组题] <= " & dr(Vars("车型"))*100

Next \'筛选

Tables("试题库").Filter = str.SubString(4)

Tables("试题库").Sort = "随机值,题型"

蓝总：Tables("试题库").Filter = str.SubString(4)  从第四个字符开始 返回所有字符串，对试题库进行赛选，这个4从得来的？