This module provides an interface for users to search one or more collections via a variety of protocols and peruse the combined results. One core application is searching library catalogues via Z39.50, a protocol for search queries against bibliographic information. The search UI is configured as a content type. This way you can make multiple search nodes, for example, one to search libraries in your city and another to search all the libraries in a region. Take a look at IRSpy to see which libraries provide their catalogs through this protocol.
The actual searching is done by a daemon called Pazpar2 that provides a web service. Since this module is a client of that service, it can search any targets exported by Pazpar2 which, in addition to Z39.50, include SRU/SRW and SOLR. The protocols are modular and Pazpar2 is GPL licensed so it would be straightforward to add more.