Parsed documentation:
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isPreferredName (http://rs.gbif.org/terms/1.0/isPreferredName)

@return [String]
This term is true if the source citing the use of this vernacular name
indicates the usage has some preference or specific standing over
other possible vernacular names used for the species.
Some organisations have attempted to assign specific and unique
vernacular names for particular taxon groups in a systematic attempt
to bring order and consistency to the use of these names. For example,
the American Ornithological Union assigns the name "Pearl Kite" for the
taxon, Gampsonyx swainsonii. The value of isPreferredName for this
record would be true.

Vocabulary: http://rs.gbif.org/vocabulary/basic/boolean.xml

Example: “True” “False”
Please help! Open an issue on GitHub if this assessment is incorrect.