好好理解帮助的代码：

http://www.foxtable.com/help/topics/2403.htm

如果类别是三位数，红色的数字就应该改为3:

Select e.DataCol.Name
    Case "类别"
        If e.DataRow.IsNull("类别") Then
            e.DataRow("编号") = Nothing
        Else
            Dim lb As String = e.DataRow("类别")
            If e.DataRow("编号").StartsWith(lb) = False \'如果单据编号前缀不符
                Dim max As String
                Dim idx As Integer
                max = e.DataTable.Compute("Max(编号)","类别 = \'" & lb & "\'") \'取得该类别的最大编号
                If max > "" Then \'如果存在最大编号
                    idx = CInt(max.Substring(3,3)) + 1 \'获得最大编号的后三位顺序号,并加1
                Else
                    idx = 1 \'否则顺序号等于1
                End If
                e.DataRow("编号") = lb & Format(idx,"000")
            End If
        End If
End Select