external_pid_file is a GUC parameter specifying the location of an optional additional pidfile.
external_pid_file was added in PostgreSQL 8.0.
external_pid_file is intended for use by the operating system (e.g. systemd) if a copy of the postmaster PID is required in a specific location.
In contrast to postmaster.pid, the pidfile generated contains just the postmaster PID, followed by a newline.
external_pid_file is an empty string.