Complex sentence: They differ from public relations departments of private organizations in that many of them typically do not engage in marketing, but solely in providing information to the public and the media as required by law and according to the standards of their profession.
Split into simpler sentences: They differ from public relations departments of private organizations in that marketing plays a more limited role. The primary responsibility of a PIO is to provide information to the media and public as required by law and according to the standards of their profession.