(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.