Excel knows more about you than you might think. Behind the scenes, all of Microsoft's Office applications store some basic information about you, including your user name. This information is set when Office is first installed (even if someone else installed it), and it usually matches your Windows user name (so if you log on as joeZhang, your Excel user name is also joeZhang).

For most tasks, it doesn't matter who Excel thinks you are. But making sure you have the right identity becomes much more important when you need to collaborate with other people. For example, if you're adding comments regarding someone else's work, you want to make sure they know that it's Tim Smith making the suggestions (and not "Excel User 1" or "SalesComputer012").