This feature is only available in Enterprise edition.
If enabled, the Search additional Outlook data files (.pst) setting will enable Identity Finder to search within all the Personal Folders Files (*.pst) that it finds within your search locations even if they are not currently attached to an Outlook profile. Normally Identity Finder will only search all PST files that are attached to existing Outlook profiles. However, if during a search it find an additional PST or if you are searching remote computers, Identity Finder will temporarily attach that PST to a temporary profile and search it. You can have Identity Finder look for detached PST files in two ways:
Only when found during file search: Identity Finder will search any PST file it encounters when it is searching for files. File searching must be turned on.
Whenever located in specified file locations (even when file search is off): Identity Finder will search for PST files in the specified locations even though File searching is turned off.
To view or change these settings, click the Settings button on the Tools and Options ribbon.