Good evening Tom
Group mode is invoked when you select a numebr of sheets (tabs) at the same time using Shift or Ctrl. Any changes made in one sheet will be reflected in all sheets, and results can be quite damaging to the unwary, and very useful to those in the know.
Try opening a new workbook, with say three sheets; highlight all tabs and then type something in A1 for it to be replicated across all three sheets.
HTH
DominicB
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