This is for folders in an ost-file that do not sync with the server mailbox. For instance, Calendar, Contacts and Tasks folders within an IMAP account. The same is true for Notes and Journal folders when using an IMAP, Outlook.com/Hotmail or other ActiveSync account. When deleting such an account, make sure you make a copy of these folders to a pst-file first.
When you are using an IMAP account, also see: Don’t risk losing your Contacts and Calendar when using IMAP in Outlook 2013.