Many items simply haven't been tested by the makers in DS and so they don't list them as being DS compatible when in fact most of them will work with the limitations already mentioned, dynamic hair and cloth and Poser shaders being the main ones. I still use Poser 4 and while almost everything these days specifies at least Poser 5, it is extremely rare to find an item that doesn't work (with the same limitations as for DS) in Poser 4.

The shaders will probably be the thing that you will encounter most often. A dress for example may have a texture that uses a one or more bitmap images (.jpg files), those should work just fine. It might also use a shader perhaps for a metallic look on the buttons - that you would have to recreate in DS, using Studio's own shader system. As suggested, it's always best to contact the maker and ask if in doubt before plunking down your money, but don't be surprised if they don't use DS and may not necessarily be  able to give you definitive information :-)

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken