The Mongo manual discuss WriteConcerns and tags but is part incorrect and part incomplete.

The 2 pages below discuss WriteConcern but mention that you can specify a tag set which should instead be that you can specify a mode, and then there should be information or a link on how to specify the mode in the rs config.

General WriteConcern page:
http://docs.mongodb.org/manual/core/write-operations/#write-concern

WriteConcern for drivers:
http://docs.mongodb.org/manual/reference/connection-string/

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Complete and correct explanation that should be included in the manual:
http://www.mongodb.org/display/DOCS/Data+Center+Awareness