Using syslog in ErrorLog directive (see core) instead of a filename enables logging via syslogd(8) if the system supports it. The default is to use syslog facility local7, but you can override this by using the syslog:facility syntax where facility can be one of the names usually documented in syslog(1). The facility is effectively global, and if it is changed in individual virtual hosts, the final facility specified affects the entire server.