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A general description is here.
Since almost all supported R versions now use UTF-8, ‘R CMD check’ no longer by default reports on marked UTF-8 or Latin-1 strings in character data. Set environment variable ‘_R_CHECK_PACKAGE_DATASETS_SUPPRESS_NOTES_’ to a false value for the previous behaviour.
Strings marked as ‘"bytes"’ now give a warning rather than a NOTE. These are better supplied as raw vectors.