Outside of pari-mutuals I would have no idea. We were "boxing in" the 1, 2, 3, 4 in the trifecta all day. Could that be it? Doubt it but who knows......
That just means you have them boxed so any combination will do. Of those 4 you need any 3 to come in the top 3 spots to win the trifecta (for a $1 trifecta). You can also box quinella's etc....