buffer-file-truename is a variable defined in `C source code'.
Its value is

global value is the same.
  • Automatically becomes permanently buffer-local when set.
  • This variable may be risky if used as a file-local variable.

Abbreviated truename of file visited in current buffer, or nil if none.
The truename of a file is calculated by `file-truename'
and then abbreviated with `abbreviate-file-name'.