Keywords Information
The following elements comprise the Keywords reference:
	Theme Keywords, mandatory subjects covered by the data set (for a list of some commonly-used thesauri, see Part IV: Subject/index term sources in Network Development and MARC Standards Office, 1988, USMARC code list for relators, sources, and description conventions: Washington, Library of Congress).
	Place Keywords, mandatory if applicable geographic locations characterized by the data set.
	Stratum Keywords, mandatory if applicable layered, vertical locations characterized by the data set.
	Temporal Keywords, mandatory if applicable time period(s) characterized by the data set.
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