These functions apply only to an X11 device with
  type = "Xlib" -- X11(type = "cairo") uses a different
  mechanism to select fonts.
Such a  device is created with a default font (see
  the documentation for X11), but it is also possible
  to specify a font family when drawing to the device (for example, see
  the documentation for "family" in par and for
  "fontfamily" in gpar in the grid package).
The font family sent to the device is a simple string name, which must be
  mapped to something more specific to X11 fonts.  A list of
  mappings is maintained and can be modified by the user.
The X11Fonts function can be used to list existing
  mappings and to define new mappings.  The X11Font
  function can be used to create a new mapping.
Default mappings are provided for three device-independent font family
  names: "sans" for a sans-serif font, "serif" for a serif
  font and "mono" for a monospaced font.  Further mappings are
  provided for "Helvetica" (the device default), "Times",
  "CyrHelvetica", "CyrTimes" (versions of these fonts with
  Cyrillic support, at least on Linux), "Arial" (on some
  platforms including macOS and Solaris) and "Mincho" (a CJK
  font).