Change tracking is really just one part of a larger feature known as workbook sharing. Workbook sharing makes it possible for more than one person to modify an Excel document at the same time. In order for this sharing to work, the workbook file needs to be somewhere that both people can access. (The most typical location is somewhere on a company network.)
Workbook sharing is a little risky (as you'll soon see), but it gives you some unique collaboration abilities that you wouldn't have otherwise. To get some perspective on how workbook sharing works, you need to understand what happens when two people fight over a workbook file that isn't shared. Read on.
Ordinarily, only one person can open an Excel workbook file at a time. Excel enforces this restriction to prevent problems that can happen when different people try to make conflicting changes at the same time and someone's changes get lost.
Figure 25-18 shows what happens when you try to open an Excel workbook that someone else is already using. Excel warns you about the problem but gives you the chance to open a copy of the document that you can save with a new name. If you need to update the version that's currently in use, you can ask Excel to notify you when your collaborator closes it and the workbook becomes available again.
Figure 25-18. Top: If you try to open a ...