The idnumber fields (found throughout Moodle) have no specific pre-set meaning, but they are often used for matching up users + courses with either external systems or, for example, CSV import.
You should be safe to use them yourself (but, unless you lock them down, they are editable via the user profile / course settings pages).
Some browser (I'm looking at you, Chrome) think that it's fun to stick values in this field for you. Autocompletion type data such as any usernames you might have saved for your moodle site! I'm constantly clearing out the idnumber field before I save. Just something to watch for.
Tim,
Please update your Moodle site - that was fixed a couple of months ago - https://tracker.moodle.org/browse/MDL-45772