Unhide worksheets with message box entry

I have some code, the purpose of which is to unhide a group of worksheets based on the data entry of one or two cells (H11 and H12). I am trying to enforce this by way of a message box entry. The problem I am encountering is that the only place I can find that it works (more or less) is in the Worksheet_Change event, and the problem with this is that every other line of code that causes a change in the worksheet will invoke the message box. I only want it to appear based on a change in range H11. If i try to to put it in the Worksheet_SelectionChange then it crashes (hard - stack overflows and other errors, including simply shutting down). I'd appreciate any advice here, or maybe there's another way to do it... The terms UnhideOpen, UnhideTSC, UnhideRoutine and UnhideDemo refer to the various worksheet groups.

Application.ScreenUpdating = False
        
Dim purpose As String
purpose = Range("H11").Value
Dim Answer As String
    
If purpose = "" Then
    UnhideOpen 
End If
        
If purpose = "Annual PM or TSC" Or purpose = "Product Clinical Demo" Or _
purpose = "Installation" Or purpose = "Incoming Inspection" Then
    UnhideTSC 
End If

If purpose = "Clinical Support" Or purpose = "Emergency Service" Or _
purpose = "Upgrade or Update" Then
    Answer = MsgBox("Will a TSC be performed during this service visit?", vbYesNo + vbQuestion, "Perform TSC?")
        If Answer = vbYes Then
            UnhideTSC
        Else
            UnhideRoutine
        End If
End If
    
If purpose = "Conference or Commercial Demo" Or purpose = "Decommission" Then
    UnhideDemo
End If

Application.EnableEvents = False

Try thiis,

Private Sub Worksheet_Change(ByVal Target As Range)
    Dim Response As Long
    Dim isect As Range
    
    On Error GoTo ExitLine
    
    With Application
        .ScreenUpdating = False
        .EnableEvents = False
    End With
    
    Set isect = Intersect(Target, Range("H11"))
        
    If Not isect Is Nothing Then
        Select Case Target
            Case ""
                UnhideOpen
            Case "Annual PM or TSC", "Product Clinical Demo"
                UnhideTSC
            Case "Clinical Support", "Emergency Service", "Upgrade or Update"
                Response = MsgBox("Will a TSC be performed during this service visit?", vbYesNo + vbQuestion, "Perform TSC?")
                If Response = vbYes Then
                    UnhideTSC
                Else
                    UnhideRoutine
                End If
            Case "Conference or Commercial Demo", "Decommission"
                UnhideDemo
        End Select
    End If
ExitLine:
    With Application
        .ScreenUpdating = True
        .EnableEvents = True
    End With
    On Error GoTo 0
End Sub

Cheers

Sorry for rush on time limited machine

Thanks for your help. I had to change the line

Set isect = intersection(Target, Range("H11"))

to

Set isect = intersect(Target, Range("H11"))

in order to get rid of a "Sub or Finction not defined error. However, when I did that and ran the code, It looks like it's going through a huge loop until an error message appears:

Run-time error -2147417848 (80010108)
Method '_Default' of object 'Range' failed

Then Excel shuts down...