The reason I chose boarding is because the parameter boarding is used at several places in OpenTripPlanner. Further, because transfer information is only useful at the moment you're moving from non-transit to transit, the value of boarding will always be the value you need in this function. However, from the perspective of the TransferTable "boarding" might be unclear, so I changed the Javadoc to describe what the parameter really says.
I will change the name of the parameter as well and add the "boarding" recommendation to the Javadoc.