(These details are for Windows: the details differ on Unix.)
  
The definition of the ‘home’ directory is in the rw-FAQ
  Q2.14: it is taken from the R_USER environment variable when
  path.expand is first called in a session.
The ‘path names’ need not exist nor be valid path names but
  prior to R 3.4.0 they needed to be representable in the session
  encoding.