Meant to ask this earlier ... but had slipped my mind

I was recently returning from Vietnam on business travel with a colleague. The immigration officer wrote "BGC" on his customs declaration form, while I had the usual entry ("1" ). This resulted in his bags being inspected by US Customs ... nothing too dramatic, this was a casual inspection which took only a few minutes and he was allowed to proceed following the inspection.

Anybody know what does BGC stand for (Back Ground Check, perhaps) and what triggers this?