De systemd para desenvolvedores III: A vingança do sistema encontramos:
The printed string in this example is logged at a default log priority of LOG_INFO1. Sometimes it is useful to change the log priority for such a printed string. When systemd parses STDOUT/STDERR of a service it will look for priority values enclosed in < > at the beginning of each line
E os números mágicos aparentemente são copiados de syslog(3)
so
$ egrep 'INFO|ERR' /usr/include/sys/syslog.h | fgrep define
#define LOG_ERR 3 /* error conditions */
#define LOG_INFO 6 /* informational */
#define LOG_PERROR 0x20 /* log to stderr as well */
Para emitir informações ou mensagens de erro de, e. um script TCL seria algo como
#!/usr/bin/env expect
puts "<3>auuugh"
puts "<6>infos"