That it’s a connecting code …
So for example the AC Flight is code sharing with United
But only for a United Airlines Pax making a connection onward
Example … someone looking to fly with United Buffalo to Heathrow
First leg is with United Buffalo to Toronto Second leg is an AC Flight Toronto to Heathrow
But … it’s code sharing with United as a regular well used route by upstate New Yorkers trying to get to London
You & I cannot book it as United … only those flying in via a United Connector can
So it’s a hidden codeshare … only listed for United Airlines Pax