Looking at that terrace I'd guess its outdoor seating for the hotel Bar. Location wise at the bottom of the alley between hotel tower blocks 2 + 4 with F.L.Y. track coming out of both launches passing overhead.
Plus brightening it up a bit there appears to a bar counter shaped thing in behind the glass frontage.
Hope to be sitting with a cocktail there before the end of the year!
EDIT:
Original better quality version of the concept art for Bar 1919.
Source
PhantaFriend's PHL Marcus has marked out on an old Google Maps screenshot the rough location of the bar beside the restaurant. Orange is the location of this terrace/outdoor seating under F.L.Y., Yellow the interior (although whether all of this space is used by the bar is unknown, I'd imagine the back half is more likely storage/kitchen facilities to be shared by the restaurant and bar)