I would actually rather translate it to Kennziffer which would mean indicator or index number in English. Especially with the examples you mentioned, Kennzeichen doesn't make any sense to me – even though the actual meanings are pretty close.
BEST_KZ for example would probably stand for "Bestell-Kennziffer" and would then be a running number identifying a single order.
If those fields you listed were named in English I would expect them to have an
_ID added – like
BEST_KZ, for example.
See also http://www.dict.cc/?s=kennziffer
Update: Just to give an impression of some "Kennziffern" in their bureaucratic habitat:
a) some job descriptions at University of the Arts in Berlin:
- "Fakultät Gestaltung | Fakultätsverwaltung | Kennziffer 2/1441/14 "
- "Zentrale Universitätsverwaltung | Referat für Studienangelegenheiten | Kennziffer 1440/14"
b) a form that enables you to ask for a "Kennziffer" ("Antrag auf Erteilung/Änderung einer Kennziffer für Antragsteller") at justiz.bayern.de – here's the PDF (check page 2!)
…but of course it could also mean "Kennzahl" or "Kennzeichen" or "Kurzzeichen" or whatever the engineer/programmer had in mind. The meaning in each of these is very similar –– however being a German part-time programmer myself I have to point out that I actually find the short form "_KZ" quite disturbing and personally would never ever use it – but instead always stick to "_ID".