Pro Pack can optionally compress/uncompress its files on-the-fly, using the same lossless compression algorithms found in .zip archives. Since most Poser filetypes are text files, you can really shrink 'em down. The compressed files have filename extensions ending with a "Z" to indicate that they're compressed. Advantage? The files are smaller, so you have more room on your hard drive. Disadvantage? Poser 4 (without Pro Pack) can't read them directly. And Pro Pack has to uncompress them before it can use them, so it takes a bit longer to load something from the Library.