The One-to-One Principle, first formulated in the earliest Dublin Core meeting, dictates that a metadata description should refer to just one resource. This principle was articulated in the recognition that most existing metadata records, in practice, combined descriptions of what might conceptually be seen as multiple distinct entities. For example, metadata records about books routinely included information such as the affiliation of an author (i.e., a property of the author), or the metadata record about a painting (e.g., "Mona Lisa") might include description of the photograph itself (a JPEG image).
The basic idea behind the One-to-One Principle became a fundamental feature of the RDF data model, which required that distinct resources be identified, distinguished, and described separately, as in the case of the original "Mona Lisa", created in 1506 by Leonardo da Vinci, and a photograph of "Mona Lisa" created in 2008 by John Smith. Accepting RDF as the foundational model for metadata allowed Dublin Core descriptions both to respect the One-to-One Principle and to transcend the limitations of "flat" single-resource descriptions.
What constitutes a "resource", hence a meaningful object of description, lies of course in the eye of the beholder. In a simple bibliography, a book may be described as a single resource, whereas a trained library cataloger might perceive the same book as the Expression of a Work, the particular version of which (Manifestation) is available in multiple copies (Items) -- in effect, as four separate resources requiring four separate, though related, metadata descriptions, potentially retrieved from four separate sources. The One-to-One Principle, therefore, is relative to the variety of subjective viewpoints in the world, where some people make distinctions while others do not, and others yet may draw the boundaries differently.
The principle whereby related but conceptually different entities, for example a painting and a digital image of the painting, are described by separate metadata records