ReverseGeocache.dat is a file created and used by the FastPictureViewer IPTC Editor reverse geo-coding feature to store the result of reverse-geocoding queries. Reverse-geocoding is the process of finding the nearest street address or place name from GPS latitude/longitude coordinates.
The database is created the first time the IPTC Editor is launched but remains empty until the reverse-geocoding feature is actually used (which requires the configuration of a reverse-geocoding provider, such as GeoNames.org or NACGeo.com).
The program caches the query results and re-uses them for subsequent queries of nearby GPS coordinates: for example if a user shoots 100 pictures on some location it does not make sense to go online and retrieve the same place name 100 times. Instead the program looks in the cache first, and if an already known place name is found within a given (configurable) radius of the current image's location then the place's name/address is retrieved from the cache instead of going online to the reverse-geocoding provider to retrieve it again (the online provider meters, throttles and charge on a per-request basis).
Since this information is stored on the computer's hard disk and constitutes potentially sensitive user data (GPS coordinates and nearby addresses), FastPictureViewer encrypts the data in the cache so only the user in question has access to said data. The encryption is automatic and invisible, and the user owning the data has full access to the file without doing anything special.
This data is encrypted using the mechanisms built in the operating system (Encrypting File System in this case) and cannot be accessed or recovered by any mean without knowledge of the user's password and actually logging-in using his/her credentials: for all practical purposes, this user-private data remains private and no one can access it except the user that created it, which is nothing but vanilla Data Privacy 101.
The program has a "Clear Geocache" command that wipes out the content of the reverse-geocache database if a user wants to erase all traces of the GPS coordinates and addresses stored in FPV's reverse-geocoding cache on his behalf.