Optional. Specifies the name or ID of the retention category to apply to the migrated PST items.
Although RetentionCategory is optional, Policy Manager must be able to obtain a retention category from somewhere.
Policy Manager takes the first retention category it finds in the following:
If MailboxDN is specified in the [PST] section, the default retention category for that mailbox.
See MailboxDN.
If ArchiveName is specified in the [PST] section, the default retention category for the mailbox that is associated with that archive.
See ArchiveName.