Create a course with a name, a short name and a course ID Number.
Make a backup.
Go somewhere else and restore the just backupped course.
The restored course is without course ID Number.
This is totally not relevant... but if you use LDAP enrolments that uses course ID Number, you can spend several hours trying to fix your correct LDAP settings.
As soon as I got this info, I also updated the moodle.docs page: http://docs.moodle.org/en/LDAP_enrolment
If you find I am wrong, please update me because I also need to revert the moodle.docs page.