The Z39.88 standard, or Open URL Framework for context sensitive services. Open URL is used heavily to connect different services related to discovery and delivery. Open URL allows users to navigate from a citation and either be direct to a full version or request a full version in a fulfillment system.
Standard for interoperability between repositories, and allows compliant repositories to have their metadata harvested by search and discovery tools.
The OAI-PMH 2 specification is the most current, as of 2017.