Yes, I can confirm what Ledhead has said is right.
I downloaded all three yesterday, and installed both DSON, and Legacy into separate dummy installs.
I am using DSPro4.5, so the correct install is the DSON file.
The Legacy was identical, but contained no .duf files. Only .dsf for DS 4.0 series. If you are using 4.0 series, then this is the one to install.
Of course, the Poser Companion Files are only to be installed into Poser with DSON/Genesis support.
Side note: Would be nice if DAZ would list this info on the download page, so one doesn’t have to guess, or trial-n-error.