It's just the meet number (usually they write it on the kids hands too). Usually the first digit corresponds to the level, but not necessarily. It's easier to enter scores in scoremaster when there's a number. And the numbers correspond to the gym. I've seen it done at NCAA meets without the number and if it's small enough it doesn't really matter but at some point, the organization is just easier and it's also easier to notice and fix mistakes. It's also on the back of the scorecard. Kind of for the judges but more the score table. Usually the judges just look at the name on the card and say it to confirm when saluting the gymnast (at least here).
Kind of hard to explain. That's just the way things are usually run here. USAG number is more than six digits and it doesn't make sense to use it. It's more like a random assignment.
