The documentation on http://docs.mongodb.org/manual/applications/replication/#replica-set-write-concern does not mention write concerns in the form of tag sets. It only very briefly mentions "w=majority".

The documentation at http://docs.mongodb.org/manual/reference/replica-configuration/#tag-sets however implies that all string values for "w" are documented there.

Right now, I can't find any documentation on how w=specialValue works.