From the Poser 5 OS X ReadMe file: Previous versions of Poser stored library file icons, and some geometry information, using the resource fork on the Mac, and .RSR files on Windows, which made sharing of files between operating systems difficult. Poser 5 eliminates the reliance on this mechanism. The .RSR files used for icons (or resource fork on the Mac) will be converted to cross platform .PNG image files the first time a library category is visited. There may be a momentary delay when this conversion happens. The original .RSR files will NOT be automatically deleted, however, they may be manually discarded if you want to clean up your directories. The .RSR geometry information stored in conjunction with .OBJ files will no longer be used. Poser 5 will read the geometry information directly from the .OBJ files only. Files converted by Poser 5 to .PNG can now be shared across Macintosh and Windows with other Poser 4 with Pro Pack or later users without having to convert the creator types or create an .RSR.