"PC only" is most commonly used by people who don't know any better. There are very few things that are really "PC only". Poser's Python GUI support differs between Mac and PC (because Poser used a package that was PC-specific), so there are "PC only" Python scripts. There are also, very rarely, products that come in self-extracting .EXE files rather than standard .ZIP files that can be expanded anywhere. There are old Poser 4 PC packages with .RSR previews that the Mac version of Poser doesn't process (ironically since they're really a Mac-specific graphics format that Poser ported to PC without the header data).

There also some Poser pose/prop files that are packaged with full PC file paths like C:Program FilesPoser 5Runtime... instead of the normal "Runtime:..." format. Of course those won't even work on a PC if Poser was installed on the D drive, or they have Poser 6 instead of Poser 5, etc.; they're not so much "PC only" as just badly packaged.

Sometimes Poser products modify copyrighted geometry using encoded packages that can't be used without special software and the original copyrighted mesh (e.g., DAZ V3). Long ago, MacConverter was the only widely available Mac utility to decode these, and it only handled a subset. Now there's a free cross-platform Java RTE decoder, and there's also an Objaction Mover for Mac.

So with the exception of a few Python scripts, "PC only" is just a bug in the package that could have been easily avoided -- or simply mislabeling.