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A general description is here.
Where characters are attempted to be plotted by ‘pdf()’, ‘postscript()’ and ‘xfig()’ 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 with a warning (e.g., dashes and Unicode minus to hyphen, ligatures are expanded, permille (‘‰’) is replaced by ‘o/oo’), although the OS may have got there first. These are warnings as they will continue to be replaced by dots in earlier versions of R.
The legacy graphics devices ‘pictex()’ and ‘xfig()’ are now deprecated. They do not support recent graphics enhancements and their font-handling is rudimentary. The intention is to retain them for historical interest as long as they remain somewhat functional.