I use a POP3 mail server, so I’ve always opted for a PST file when setting up a new account in Outlook. PST stands for Personal Storage Table. Outlook uses it to store all your email items, contacts, and calendar. It may be designated Outlook.pst or something like firstname.lastname@example.org. Either way, it resides in the Outlook Files folder of your Documents folder.
I recently installed Office 365 on a laptop and discovered that instead of PST I had an OST file. OST is the initialism for Offline Storage Table.
Because PST files are local, you can copy them to other PCs that have Outlook that uses a PST, provided you change the transferred file’s name to the existing one before replacing it. This isn’t possible with OST files, which are copies from the exchange server. When you disconnect from the Internet, the local copy gives you continued access to all the files. Once the Internet connection is restored, the local file is synced with the server’s file.
Unlike PST files, OST files cannot be backed up.