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A general description is here.
Where characters are attempted to be plotted by ‘pdf()’ and ‘postscript()’ which are not in the selected 8-bit character set (most often Latin-1) and the R session is using a UTF-8 locale, the warning messages will show the UTF-8 character rather than its bytes and one dot will be substituted per character rather than per byte. (Platforms whose ‘iconv()’ does transliteration silently plot the transliteration.)
In a UTF-8 locale some transliterations are now done without warning (e.g., dashes and Unicode minus to hyphen, ligatures are expanded).