Windows does a lot of work in the background when you shut down, restart, or sign out of your PC. The process ensures all your work and application data is saved before the hardware powers off.
Windows Checks for Logged in Users (on Shut Down)
When you tell your PC to shut down or restart, Windows first checks to see if any other Windows user accounts have active sessions. This occurs when you lock your Windows session and sign in with another user account before signing out first.
If Windows does notice another user hasn’t properly signed out yet, you’ll see a “Someone else is still using this PC” message. That other user could lose any unsaved data in open applications if you forcibly restart. It’s usually a good idea to stop here and let the other user sign in, save their work, and log out before shutting down.
Windows lets you click “Shut down anyway” if you’re sure the other user doesn’t have any open work to save. This will forcibly sign the other user account out, closing all their open applications. Any unsaved data will be lost.
If you’re the only signed in user, you won’t see this message and Windows will go straight to the next step.
Windows Tells Programs to Save Their Work and Close
Before actually signing you out of your PC, Windows tells all your open programs to save their work and close. This also occurs when you shut down or restart your PC since signing you out is a necessary part of the shutdown process.
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Source : What Exactly Happens When You Shut Down or Sign Out of Windows?
