Data entry sequence

Option Explicit
Dim ChkErr      As Long
Dim Txt1        As String

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    On Error GoTo ErrLne
    Dim L1          As Long
    Dim L2          As Long
    Static RngLast  As Range
    If Not Application.Intersect(Target, Range("A1:ZZ65000")) Is Nothing Then
        If Not RngLast Is Nothing Then
            If Range(Target.Address).Column = 1 And Range(RngLast.Address).Column = 1 Then
                If RngLast.Address <> Target.Address Then
                    'MsgBox "Lost focus " & RngLast.Address & " " & Range(RngLast.Address).Row & " " & Range(RngLast.Address).Column & " " & RngLast.Value & " Txt1=" & Txt1 & "(" & Len(Txt1) & ")"
                    If Txt1 = "" Then
                        If Not (RngLast.Value = "") Then
                            L1 = 0
                            For L2 = 1 To 65500
                                If Val(ActiveSheet.Cells(L2, 2)) > L1 Then L1 = Val(ActiveSheet.Cells(L2, 2))
                            Next L2
                            ActiveSheet.Cells(Range(RngLast.Address).Row, Range(RngLast.Address).Column + 1) = L1 + 1
                        End If
                    Else
                        If RngLast.Value = "" Then
                            L1 = ActiveSheet.Cells(Range(RngLast.Address).Row, Range(RngLast.Address).Column + 1)
                            ActiveSheet.Cells(Range(RngLast.Address).Row, Range(RngLast.Address).Column + 1) = ""
                            For L2 = 1 To 65500
                                If Val(ActiveSheet.Cells(L2, 2)) > L1 Then ActiveSheet.Cells(L2, 2) = Val(ActiveSheet.Cells(L2, 2)) - 1
                            Next L2
                        End If
                    End If
                End If
            End If
        End If

        If Range(Target.Address).Column = 1 Then
            Txt1 = Target.Value
            'MsgBox "Got focus " & Target.Address & " " & Range(Target.Address).Row & " " & Range(Target.Address).Column & " " & Target.Value & " Txt1=" & Txt1 & "(" & Len(Txt1) & ")"
        End If
        Set RngLast = Target
    End If

    Exit Sub

ErrLne:
    ChkErr = Err.Number
    MsgBox "Error " & Err.Number & " : " & Err.Description, vbInformation
    Resume Next

End Sub