From GNUstepWiki

NSDataLink (part of AppKit) defines a mechanism for embedding "live" data in a document, e.g. an updated image in a word processing document. If one were to change the image, the version in the document would reflect those changes immediately, without needing user intervention.

This functionality is not present in Cocoa, though the open source LinkBack mimics that behavior.

OpenStep specification of NSDataLink