The CI_OnlineResource element is used in a variety of contexts in ISO19115 content. This complex-content-element provides a linkage to some online resource related to the metadata or the described context resource. It is thus analogous to atom:link (IETF RFC-4287) or the link element described in IETF RFC-5988.  For links that locate online documents accessible using standard browser and file type resolution technology, the link can be as simple as a single URL element that retrieves a representation of the resource. There are many other kinds of related resources that the onlineResource element may point to, including web services that provide access to a dataset resource, metadata for related or source resources, specifications for standards or extensions to standards. 

For the USGIN, it is desired that such links are accompanied by sufficient description that the linked resource can be accessed and provided to the metadata user automatically, with little or no operator intervention other than clicking on a link in a presentation of the metadata. A content specification for such machine-actionable links is discussed in a separate discussion paper. This approach essentially considers the metadata as a sort of hypermedia in which the CI_OnlineResource elements define the 'affordances', or actions that are available to a client.

NOAA metadata wiki discussion of CI_OnlineResource

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Extracted text on CI_Online Resource from USGIN metatadata profile