The NCIP standard/protocol is used primarily in the US and Canada, and is the standard that is used to exchange circulation information between systems such as self-service circulation, ILL systems, and other systems that need to pass/exchange data about item availability and other circulation type information.
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.
The Pubmed ID (PMID) is a unique number assigned to each article/paper indexed in Pubmed, which is a comprehensive index of literature in health sciences and medicine. The PMID is used heavily in discovery and linking between systems. Additionally, the PMID is used by researchers to find and retrieve articles.
Digital Object Identifiers are standards for unique identifiers for digital objects, and is an ISO standard: ISO 26324. The DOI is used primarily to identify articles from journals, book chapters, books, and other objects.
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.