Clarrification of @href/@keyref on

The 1.2 reference entry for <data> says for the @href attribute: "Provides a reference to a resource. See The href attribute for detailed information on supported values and processing implications."
However, I can't find any discussion of what it actually means to have <data> associated with a resource.
Do we say anywhere?
If not, what is the intended implication?
Is the associated resource the metadata value?
Thanks, Eliot

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Eliot Kimber Senior Solutions Architect, RSI Content Solutions "Bringing Strategy, Content, and Technology Together" Main: 512.554.9368 www.rsicms.com www.rsuitecms.com Book: DITA For Practitioners, from XML Press, http://xmlpress.net/publications/dita/practitioners-1/