The multiple users setting allows you up to 3 different UserIDs to use one installation of PokerStat. You can only change this setting if you have no database opened.
For example, let's say both you and your spouse use PokerStat with different UserIDs. To setup both UserIDs on one computer, you would first open PokerStat, make sure no database is open, and go to the [View] menu, and choose [Multiple Users (rarely used)]. Then choose one of the three settings.
Each setting is like a separate installation of PokerStat, with it's own registration information, hands filter settings, program settings, etc...
Note that you can also change settings by pressing the shortcut key. Hold down CONTROL, and press 1, 2, or 3 to choose multiple user settings 1, 2, or 3, respectively.