You'll use this at the airport where you clear US customs, not MCO but Ottawa since they have pre-clearance. The app is really usefull in Montreal, to skip part of the US Custom lineup, which can be horrible! I used it twice last year. Once I was travelling by myself, spotted an ex-colleague in the other line. Finally met him on the US side and he confirned he spent 1+ hour in line (I spent 15 minutes). 2 years ago we waited over 2h with the kids without MCB...