To minimize downloads to the Vault Cache, the Outlook Add-In regularly searches the mailbox for any items that are due to be archived soon. It automatically adds these items to the Vault Cache. This feature is called preemptive caching.
The Vault Cache advanced setting controls whether an offline store is required in Outlook for Vault Cache to be enabled. If a user does not have an OST file, Enterprise Vault cannot perform preemptive caching.