Librarians share things a lot. Not just with their local communities but with other communities. So when they send off their resources (or authorise access to an electronic resource) to other libraries to lend to their local communities there are a sequence of transactions that happen in the home and away systems, and maybe some intermediary systems to facilitate all this goodness.
This is where NCIP comes in.
“This standard defines a protocol that is limited to the exchange of messages between and among computer-based application to enable them to perform functions necessary to lend and borrow items, to provide controlled access to electronic resources, and to facilitate co-operative management of these functions.” – abstract ANSI/NISO Z39.83 (Part 1) - [NISO] Circulation Interchange Part 1: Protocol (NCIP).