which file/s to backup?

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Outlook version
Outlook 2016 64 bit
Email Account
I'm puzzled. I'm using Windows 10 and Outlook 16. I first started using this on a laptop runnin Windows 8.1 and set up accounts as IMAP. When a drive failed I restored from a backup and learned that IMAP (.ost) doesn't contain calendar, contact, or category information. I created a new profile, this time setting the accounts to the pop server and using a new pst file. Wanting to be sure that my calendar, categories and contacts woul be backed up when the pst was, I checked the time stamps of the files. I open Control panel>mail and choose the data files tab, all data files are .pst. The same is true if I go through Profiles. All are located in C:\Users\Myloginname\Documents\Outlook Files\. what is puzzling is that there is also an.ost located in C:\Users\Myloginname\appadata\Local\Microsoft\Outlook. The modified time stamp is current for the ost and pst. Two related questions:
1) Why is there an ost if all accounts are pop and use a pst file as the data file? (I deleted it and it was recreated)
2) If I ensure that the pst is in a backup, can I be sure that restoring it will also restore my calendar, categories, and contacts?
I don't know why outlook created and is loading the ost but its not using it if you are using pop3. If you used autoaccount setup and then switched to pop3, it could be some leftover artifacts in the profile. It should either be 256KB, 512KB, or 16,424KB. Those are all empty sizes.

Our recommended configuration is IMAP with a pst set as default - this is the only supported IMAP config in older versions - and gives you the benefit of IMAP but without the risk of losing calendar and contacts.
I was puzzled by it, too. But I've had a stroke and had a fire destroy everything, then the laptop that was supposed to get me through until the house was rebuilt, alerted that the harddrive failed an internal test. It was less than 3 weeks old! So, I just chalked it up the Outlook issues to stress and old age! Looking on the positive, I saw an opportunity to build a desktop with the power that I needed and a clean install of window 10 and all my programs. I imported the .ost from an image of the failing drive and soon discovered that all of my calendars, categories, and contacts were missing. Always looking for the easiest way, I followed your directions for creating a .pst. But what I hadn't realized was that each account now had it's own inbox. I accounts were using their own data file. I just added the contents of those inbox and other folders to my primary inbox and then deleted all but the primary account. Once again, I set up each account, this time making sure that they all used the one primary account's data file.
Now there is no .ost file and the calendar, categories, and contacts are as before.
Instead of spending time trying to figure out what was wrong, I should have just acknowledged that I might never "figure it out" and just experimented till it was the way that worked for me.
As a quick follow up. Losing everything in the fire made me look at "cloud" backups. The programs that held the most promise from a security and cost standpoint were not able to backup .ost files, but could take incrementals of open .pst files. And thus the saga began....
If you are using imap, you only need to back up the pst that holds the calendar and contacts. While you can't connect the pst outlook when its stored in the cloud, you can store a backup copy of the pst in cloud storage.
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