Hi Frank,

If I understand right, if AngtType = "SS member", you want to set NoInd = 0.

And you want to reuse the number if you change AngtType from "Anggota
Jemaat" to "SS member".

Here is your modified sub and a function to generate the next number:
(Watch for line wrap)

'--------code beg -------------------
Private Sub AngtType_AfterUpdate()
  If AngtType = "Anggota Jemaat" Then   '  1
'      NoInd = Nz(DMax("[NoIn]", "bukuangkby")) + 1 ***
     NoInd = GetNextNumber
     If Not IsNull(Me.[AngtType].OldValue) Then   '2
        If Me.[AngtType] <> Me.[AngtType].OldValue Then   '3
           If MsgBox("Anda telah merobah!!, apakah sengaja mau merobah?",
vbYesNo + vbDefaultButton2 + vbQuestion) = vbNo Then   ' 4
              ' Undo the changes
              Me.AngtType = Me.AngtType.OldValue
           End If   '4
        End If   '3
     End If   ' 2
  Else    '  1
     'If you want the field NULL, uncomment the this line
     ' NoInd=NULL
     ' and comment the the this line
     NoInd = 0  '***
  End If   '1
End Sub

Public Function GetNextNumber()
'Function to generate the next number
  Dim sSQL As String
  Dim r As DAO.Recordset
  Dim s As Byte

  'create a recordset
  sSQL = "select top 1 bukuangkby.noin + 1 as NN,"
  sSQL = sSQL & "  (select min(t.noin) - 1 from  bukuangkby as t where
t.noin >  bukuangkby.noin)"
  sSQL = sSQL & "  from  bukuangkby"
  sSQL = sSQL & "  where not exists (select t.noin from  bukuangkby as t
where t.noin =  bukuangkby.noin + 1)"
  sSQL = sSQL & "  AND bukuangkby.noin< (select Max(s.noin)+1 from  
bukuangkby as s)"
  sSQL = sSQL & "  Order By noin;"

  Set r = CurrentDb.OpenRecordset(sSQL)
  If r.RecordCount = 1 Then
     r.MoveFirst
     'return the number
     GetNextNumber = r.Fields(0)
  Else
     ' no number - let them know
     MsgBox "ERROR! Number not generated!!"
  End If

  'clean up
  r.Close
  Set r = Nothing

End Function
'--------code end -------------------

HTH