These are pre-designated spots on the airport where aircraft contact Ground control. In Cleveland, we use the "spot" for taxi out of the ramp (we have 6 "spots"). These are all arranged with the control tower and Continental (the largest operator at the airport and they also control most of the ramp).
In this case, it sounds like aircraft are being taxiied to a particular spot where they will contact the ramp tower for taxi to their gate. I know earlier that PHL had flow control to the airport.