Don't use the resource fork of a file for non-resource data. Parts of the system (including the File Manager and the Finder) assume that if this fork exists, it will contain valid Resource Manager information.
PBOpenRF was provided to allow copying of the resource fork of a file in its entirety, without Resource Manager interpretation. Do not use it to open "another data fork."
The File Manager assumes that the first block of the resource fork of a file will be part of the resource header, and puts information there to aid in scavenging. Note that this means that if you copy a resource file (opened with PBOpenRF), the duplicate may not be exactly like the original.
Further Reference: