This policy setting controls whether Outlook creates a new OST file when you upgrade to Outlook 2013. The new OST file uses less space on the disk. When a new OST file is created, the contents from the previous version of Outlook are downloaded from the Exchange Server.
If you enable this policy setting, Outlook continues to use the existing OST file created by the installed earlier version of Outlook.
If you disable or do not configure this policy setting, when you upgrade to Outlook 2013, a new OST file is created, and the contents from the installed earlier version of Outlook are downloaded from the Exchange server.