The Combined Search tab is seaching all of the contents of the CATALOG & REPOSITORIES tabs, PLUS:
The HathiTrust is a partnership of major research institutions and libraries working to ensure that the cultural record is preserved and accessible long into the future. The HathiTrust Digital Library brings together the immense collections of partner institutions in digital form, preserving them securely to be accessed and used today and in future generations.
The Primo Central index is a mega-aggregation of hundreds of millions of scholarly e-resources of global and regional importance. These include journal articles, e-books, reviews, legal documents and more that are harvested from primary and secondary publishers and aggregators, and from open-access repositories.
See the video below for information about Primo Central.