What is the difference between an institutional repository and a subject matter repository?

Open access repositories may be institutionally-based (e.g., universities or laboratories) enhancing the visibility and impact of the institution and its faculty (e.g., Penn Scholarly Commons at the University of Pennsylvania), or they may be centralized, subject-based collections like the economics repository (e.g., RePEc Research Papers in Economics), the physics repository (arXiv), or the Aquatic and Marine Science repository (Aquatic Commons), a thematic digital repository covering natural marine environments. More information can be found here.