PSF Subformats
PSF initially stood only for "PlayStation Sound Format", but with the addition of the PSF2, SSF (Sega Saturn Sound Format), DSF (Dreamcast Sound Format), USF (Nintendo Ultra 64 Sound Format), QSF (Capcom Q-Sound Format), GSF (Game Boy Advance Sound Format), and 2SF (Nintendo DS Sound Format) subformats, the more generic backronym "Portable Sound Format" was developed. As a result, PSF and PSF1 interchangeably refer to PlayStation sound data files.
Read more about this topic: Portable Sound Format