Steve, marking Shane's reply as "answer" is incorrect again, even if you think it is helpful.
#1 thing to remember about MS products is that technical and user terminology historically get mixed together in common usage. On top of that, MS has a long history of recycling names/terminology. "Outlook" is a common example. There are several historical and current "flavours" of Outlook with different features right now: Outlook on the desktop, Outlook on the internet, Outlook on smartphones etc. "Outlook" in Consumer and Buiness Office use different email server technology in the background on the same user application. The constant give and take between users, technologists and MS Marketing leads to the potential for massive user confusion. This current case is a small example.
In Windows an "instance"/"session"/"task" represents a discrete program task running in Windows. If you stop a "task" in windows any file open in that task is also killed (not "closed"). In my earlier examples I show that Windows is running Excel in up to 4 separate Windows "Tasks". One of which has 3 separate workbooks open: Workbook3, Workbook4 and Workbook7. If I was to "kill" that specific task in Task Manager (aka Task Mangler <g>) All three of those workbooks would disappear from the screen at the same time WITHOUT saving the files. AKA a manually triggered "Crash"
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Windows opens all files in a single application "Task" to reduce resource use like RAM, and CPU to a lesser degree.
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The "Task" bar is a bit of a misleading name. It does not explicitly show discrete "Windows tasks". It shows files and applications as separate USER tasks. But as it runs out of space, it will "lump" files in the same applications into a single "task" in the bar. But those files can be running in separate Windows tasks.
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I explicitly start Word in separate sessions because I've lost too much time when Word has crashed with several files open, forcing me to recreate work. So now, when I want to open a new file I prefer to start a new "task" from a shortcut on my desktop rather than using File Open, which would open the file in the same Windows task, exposing me to the opportunity to cuss at MS when Word crashed and lost work in multiple files. I may be slow, but I do learn,eventually.
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Here is a new example. I have 5 Word files open in 4 separate Windows tasks. The files show up separately in the vertical taskbar on the right side of my HD screen.

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Forcing all of those open files onto my lower resolution laptop screen forces Windows to show all of the files under a single "Task" on the taskbar. But they are still running in separate Windows tasks!
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If I was select the highlighted task in Task manager, and used the "End Task" button to "kill that task, the 2 files in it would close, but the other 3 would still be open.
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(Sorry, I just realized that using a yellow highlight on the blue icon in the taskbar makes it icon look green, like Excel, a little misleading).
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<CTL><F4> and <ALT><F4> do NOT do the same thing in Windows applications as Shane contends!
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<CTL><F4> closes one file at a time. If I used it in the session with the "@A Answers" and "@W Tips Word" files open it would only close the current file. The other one would not be affected.
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<ALT><F4>, would close both files, prompting you to save any unsaved changes in both files.
The other 3 files, in the 3 other Word "tasks" would not be affected in either case.
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Both of Shane's replies are wrong.
For future users, I would appreciate it if you "unmarked" Shanes replies as answers. If you can't, let us know and I'll ask the moderators to do it for you.